Transcript for:
GCSE Chemistry Key Content

okay folks good afternoon um I hope that you've had a a successful day of revising or doing exams whatever you've been doing um this is going to be a live stream going through everything that's going to be in GCC Chemistry Paper 2 um in a bit more detail than the pre-recorded video that's on my channel um so just to give you a heads up in case you don't know what else is available on the channel a few other things um if you don't have enough time to stick with us for this whole live stream this afternoon because this is going to be an extended version um there is a pre-recorded version that's a little bit shorter it's about an hour which you can find at this address here um if you're wanting to make sure that you've covered all the key facts then this address here is a bunch of um recall questions for each of the topics um these links are case sensitive by the way so you need to get the capitals in the right place um and then for each set of those questions there's a video that goes with them um which tells you all the answers um I've also made a new video this week which is all the required practicals for this topic sorry for this paper um and then also for each individual topic there are specific summary videos so if you're sort of particularly thinking that you want to revise organic chemistry or atmosphere or whatever it is without going through the whole paper um then those videos are there for you so this live stream is going to be everything for triple science and combined science foundation and higher um and if you keep an eye on the tags at the very top of the screen um anytime that there's content that is only for higher tier or only for triple scientists it does say up there and I'll try and call it out again as well um so if you're taking combined science or if you're taking foundation um then you know which bits you kind of don't need to revise with us so the very first topic that we need to cover is rates of reaction so the rate of a chemical reaction is its speed and we can measure that in two ways either how quickly it uses up reactants or how quickly it makes products and there are two formula that you can use to calculate this but they both have the same format so either it's reactant used or it's product made but either way it's divided by time and this is a really important thing that you can't calculate rate without a time so if you're describing any um reactions any methods you need to be talking about timing that reaction and recording that um so these can be in um you can have the rate as grams/s or centimeters cubed/s and then also if you're taking higher tier you could also be asked to calculate in moles/s so just for an example of how we would use this and if we had a reaction that produced 20 cm cubed of gas in 2 seconds then I would do 20 / 2 and that would give me 10 cm cubs which is the mean rate for that reaction now one thing to watch out for is that our units here are centime cubed so in other words a cime by a cime by a cime right that's part of the units it's not part of the number so when I do this calculation I don't need to cube 20 i just use 20 and the cubed sign is just part of the units if I wanted to collect data in terms of a mass change then I would need to carry out my reaction on a balance which is this um so you might be tempted to call this a scale but our proper science word for it is a balance that's what we want to call for it so this is going to work for reactions that release a gas as one of the products um because therefore the mass of the reaction um is going to decrease so that could be something um where we've got some metal with some acid in which case it would release hydrogen or actually I've just realized on this diagram I've labeled it as um marble which is calcium carbonate so that would release carbon dioxide but regardless if a gas is being released then we could measure the mass and that would allow us to calculate a rate for it i record my um data in a table like this u it's important that the units are in the headers so I've got grams written up here i wouldn't have grams written down here that would be a bad thing so I only want the units up here in the header um and it's also really important that in my table I include um zero for a time so I find out what was the mass before the reaction had started and that's also going to be true if you're asked to plot some data on a graph really commonly um people miss out the data point for where the time is zero and they just sort of think it's not important so make sure that you are including that so what we're actually interested in is the change of mass and it's important that we're always um comparing it to the very start so the first one of these is easy because the mass hasn't changed yet and the second one is also easy because you can see it's gone down by 0.07 g but the place that people start making mistakes is with the next line of the table because what we actually need to do is compare always to the start so lots of people are going to accidentally compare to the one above and they're going to say 0.05 but that would be wrong we don't want to do that we want to compare to the start so we'd have 0.12 and then 0.16 and yada yada yada so if you are measuring um the mass change for a reaction so that you can calculate the rate um your overall mass change is always going to be going up because you're not going to suddenly put some of that gas that escaped back into the reaction um so if your numbers don't keep on going up then something's gone wrong um alternatively instead of measuring the mass change you could collect the gas as it's released and then um measure the volume there so there are two different options that you can do um you can use a gas syringe which is this piece of equipment that you can see in my picture here um and that's probably your um your best way of doing it because it's going to give you really accurate data um and it's sort of relatively easy to use but you may never have used one in school because um they're quite fragile they're usually made of glass and they're quite expensive to replace um and frankly a lot of schools don't trust their year 9 with them um so what you're more likely to have done is this is called collecting gas over water and it's where we've got um a measuring cylinder that's full of water and then as the gas um comes out of this delivery tube here so this is my conical flask where my reaction would happen um the gas would come through that delivery tube and that delivery tube if I was actually in the middle of doing the reaction would go up here into the measuring cylinder and as the gas came out it would displace the water it would push the water out so I'd end up with a gas bubble forming up here and as the bubbles came up that bubble would get bigger and bigger and I can measure the um volume by reading it off the measuring cylinder so that's your alternative you need to be able to plot rate data onto a graph and also interpret graphs that the exam board have given you um so in case you weren't aware for GCSE chemistry 20% of your marks are for math skills and those have to be math skills that are hard enough um if you're taking higher tier science the math skills have to be hard enough to be included in the GCSE maths foundation tier paper if that makes sense so this is one of the math skills they could test um can you plot some data on a graph um so I'd start off and as we say we've got to make sure that we've included that point at 0 0 and then we would add the rest of these in and I would draw a line of best fit through those it's important to remember and I know this is different to what they teach you in GCSE maths um your line of best fit should reflect whatever your data is doing so here you can clearly see that my data is curving and therefore my line of best fit also needs to curve so the gradient of a graph of mass change over time or volume change over time can tell you the rate of the reaction and essentially the steeper it is the faster that reaction is going now we can actually work out some some numbers for it but also just qualitatively we should be able to say that's what's going on there so in this instance the reaction that's represented by the purple line has got a faster rate than the blue reaction um because the line has a steeper gradient so I've just realized some people might be color blind um but this line on top is faster and if they asked me how I knew I could say well at 10 seconds um that had changed by five whereas this has only changed by about um 2 and a half now you can also look at a curved graph and make similar conclusions so here we can say that my reaction is going faster at the start which is what we expect basically every reaction you meet at GCSE is going to go faster at the start because there are more reactants available and then gradually as time has gone on um it's slowed down um you could be asked to actually calculate the gradient of that graph so obviously from your math lessons um you know that we can calculate the gradient by doing the change in y over the change in x um or you might remember rise over run or something like that but if I wanted to work out the average rate of reaction here I would say okay it's used up five grams i can see that here and it's taken 10 seconds to do that so again I just use that same formula um and I find that 5 / 10 is 0.5 now if you've got a curved graph um firstly we can calculate the mean rate of reaction the average rate of reaction um and to do that we just find out how much has that reaction used up and we do exactly what we did before but sometimes that's not going to be quite enough sometimes you're going to want to know what is the reaction at a particular what is the rate of the reaction sorry at a particular time so like what is the rate here or what is the rate here we've said that that rate is changing over time we can see that the graph is getting less steep but it's very very hard to work out the gradient of a curve so we don't what we do instead is we draw a line called a tangent and that tangent touches the curve in just one place and therefore the gradient of that curve will be exactly the same as the gradient of the tangent so your tangent is going to give you the most accurate result if it's as long as possible so I could have drawn a little tangent here that was just sort of that long um but that would make it very hard for me to accurately measure how tall it was and how wide it was so you want to make this tangent as long as you possibly can and you want it to only touch your curve in one place um if you don't have a transparent ruler it's a good idea to put your ruler on top of the curve oh sorry that was terrible drawing rather than underneath the curve cuz that way you can still see the curve so anyway if I draw that tangent there at 4 seconds I can then say okay so the mass has changed from 2 up to 9 which is seven um and that's taken 10 seconds to have so therefore my rate is 0.7 um and then here at 7 seconds we said it's a little bit flatter again I've drawn a tangent this time it's gone up from sort of 4 and 1/2 to 6 and 1/2 i know that my graph is not um super easy to read but obviously the one in the exam is going to have um far more um sort of grad uh gradations so that you can actually see what's going on a little bit um so you'll have a a clear idea of what's going on there so then we move on to collision theory and collision theory is this idea that chemical reactions will only happen if the particles that are reacting collide with each other so that means bang into each other and also if they have sufficient energy and we call that minimum amount of energy that they have to have the activation energy so there are five different factors that can affect the rate of a reaction so we've got pressure concentration surface area temperature and catalyst you can increase pressure in two different ways one is by increasing the number of particles in a container so if you imagine about um blowing up a balloon or blowing up a bike tire and the other is by making the container smaller because the pressure is caused by the particles bouncing off the walls of the container they're in so if they're in a smaller container they're going to collide with the walls more frequently um so we would say that increasing the pressure will increase the rate because the particles will collide more frequently and frequently is a super important word um you could also say more often or you could say more times per second but you can't just say more um and the easiest way to think about this is um I could say to you oh um I'm faster than Usain Bolt because I ran 5 km yesterday and he only ran 100 m and obviously that's complete rubbish because even though um he only ran 100 meters he did it in 10 seconds whereas my 5 km took me a lot longer than 10 seconds so it's not just enough to say that the particles are colliding more because they might have taken a longer time to do it it needs to be more frequently more often more times per second um so next up concentration so this is very similar to pressure but it applies to solutions rather than gases but it's the same idea if the concentration increases we have more particles in the same volume so therefore they're more likely to collide they collide more frequently that important word again and therefore we're going to have a higher rate of reaction next up is surface area so as you cut an object into smaller pieces you increase the surface area um and often that leads to a lot of confusion because people are thinking well it's one tiny particle how can it have a bigger surface area so we don't mean the one tiny particle has a bigger surface area we mean if we've chopped up a big piece of say metal into lots of little bits the surface area of all those bits put together and the easiest way to visualize this is I know this is in 2D but if you imagine that this here is a piece of metal so maybe this is some magnesium or something and I'm putting it in some acid and the acid can react with all these bits around the outside but it can't get to here in the middle cuz it's sort of shielded whereas if I cut that magnesium into some smaller bits then suddenly all these internal faces can also react so again we're going to say that our particles collide more frequently and therefore the rate of reaction is higher so next up we need to think about temperature um so all of the particles that are in a reaction have a certain amount of energy and we've said that they need to have a minimum amount called the activation energy in order to react so um so this line here this blue line represents the activation energy and in my reaction I've got six particles and you can see that some of them have quite a lot of energy and some of them have not very much so in this reaction the three particles that have more energy than the activation energy would be able to react if they collided but the other three even if they collided no reaction would happen so if I heat up a reaction I am giving more thermal energy and the particles are going to use that as kinetic energy so they're going to move faster but regardless what they have is more energy so they're going to move faster they're going to collide more frequently but also if we give them all a bit of energy now four of my particles have enough energy they have the activation energy where before it was only um it was only the three of them um so that's why um increasing the temperature is going to increase the rate of reaction there are two parts to it um there's the fact that they move faster they have more energy so they collide more frequently and the fact that a greater proportion have got um the activation energy and therefore um uh therefore if they do collide then um they're actually going to react so I got distracted cuz I just tried to look at the comments and I was trying to read and talk at the same time which is very silly of me okay so then our last way that we can speed up rates of reaction is by introducing a catalyst so a catalyst is a chemical it has to be a chemical i cannot tell you how many times um people have written that high temperature or energy is a catalyst and it's not it has to be a chemical that you're adding in these chemicals speed up the rate of reaction without being used up or permanently changed themselves so you can get that catalyst back after the reaction is finished um and enzymes are an example of a biological catalyst so enzymes exist within cells and they speed up reactions within that cell or within that organism so regardless of whether we're talking about chemical catalysts or biological catalysts um they provide an alternative pathway that has a lower activation energy so if here is me and I want to go visit my friend it's going to take me a certain amount of energy to climb up over the mountain and if I don't have that energy if I'm feeling a bit tired today then I can't get there it just can't happen and in the same way if a particle doesn't have enough energy it just can't react whereas if I have um an alternative pathway that's around the bottom of the mountain it's not changing how much energy it takes um to go over the mountain it's just giving you an alternative weight that takes less energy um and if you were drawing an energy profile diagram which we did a lot of in unit 5 paper one um we would mark the activation energy between the um the reactants and the transition state it's always going there um but if there was a catalyst involved then we would see that the activation energy is smaller um and I think certainly in the triple paper we didn't have any reaction profiles um in paper one so there's a reasonable chance of them coming up in paper two okay quick water break while I catch up on the comments okay so in answer to the question about whether this covers everything even the pre-recorded video um which is sort of 60 minutes long covers every point of the spec i haven't skipped any sections but what I obviously have to do is do some of it in not a huge amount of detail because um you know if it's taken you two or maybe three years to be taught this course I can't possibly cram it all in um so this is going to be in quite a bit more detail it's probably going to take me two two and a half hours um but evidently like I can't teach you everything that you've been taught in the last 2 or 3 years um so this is covering everything in um combined science and um triple um covering all of it but um there is there are headers at the top and there haven't actually been any yet because we haven't covered anything yet that isn't just in combined foundation but when we do get to that bit then um it will say at the top and also I'll call it out uh also no this is not pre-recorded um I did pre-record the physics one and then you all gave me such a hard time that I'm doing it live that's why there's a lot more ums um how long is combined this well I'm not doing combined and then triple at the end i'm doing it in the order it's in in the spec because there are triple bits mainly in units seven and 8 and 10 um so I guess if you're doing combined then the last sort of 20 minutes or so u which is triple for unit 10 you'll be able to skip off and not stay for um but it is kind of integrated and there's not a lot I can do about that yes my last one was pre-recorded because it was physics um but uh yeah people were not a fan about this um and is it true that AIDS is spread through touch not hugely relevant cuz we're not doing biology paper one um but if you don't have bloodto blood contact or uh bodily fluid to body fluid contact I think you're going to be okay right let's crack on then so first required practical we've said already that the required practicals make up 15% of all the marks available so if you find that you have one thing that you can revise it needs to be the required practicals so the required practical for unit six investigates what happens to the rate of reaction when we alter the concentration of one of the reactants it's important that you're only going to be changing um one of them because obviously if we um change the concentration of both reactants if it's two solutions then um that wouldn't give us valid data in our primary school language we would say it wasn't a fair test so this is a two-part investigation because you have to investigate one reaction that collects gas so like we mentioned before here's my gas syringe um and then we've also got a second um a second practical where we're investigating turbidity and turbidity means cloudiness but we'll worry about that in a second um yes iron wall is a chemical catalyst yes it is for foundation too um so um if we're going to collect gas it needs to be a reaction that will release gas so um it's usually going to be magnesium in acid or marble chips in acid so there are two ways that we can do this and we did mention these already you can either collect gas over water by using an upturned measuring cylinder or you can use a gas syringe um whichever method you're um describing you need to make sure that you're talking about taking readings at regular time intervals so measure the volume of gas that is produced every 5 seconds or measure the volume of gas that's produced every 10 seconds or whatever it is um because this is a a required practical because this is an investigation you need to make sure that you are clear and you can describe what are your independent variable your dependent variable and your control variables so independent variable is the one that you are changing which in this is going to be change in concentration usually of some acid although it could be not an acid um dependent variable is whatever you are going to measure and observe so here it's effectively the rate and then control variables are all the things that you going to keep the same i am a control freak so I always have my lilac PowerPoint i always have my classes sat in my seating plan i always use the same font um because I am a control freak so um if we were going to do different concentrations of acid um and we were putting in some magnesium then I would change the concentration of the acid but I would keep the same the volume um and I would keep the same the mass of the magnesium um you shouldn't ever we quite often see people writing things about oh I'll keep the conicle flask the same or something um the equipment that you're using is not going to be a control variable um because it needs to be something where if you changed it it would change the results um and if I just washed up this conicle flask and used another one then that wouldn't um that wouldn't affect um the results that I'm going to get um a little vocabulary check thing you've all done chemistry paper one you all hopefully know that amount means the number of moles um and very often when you think the word amount you don't mean what chemists mean by amount so by far the easiest way particularly with paper two is to just ban the word amount from your vocabulary if it is a solid talk about the mass if it is a gas or a solution or a liquid um then talk about the volume just do not use the word amount um because so often people say it when it's not what they mean and then they can't have a mark okay so then the second part of the required practical um is a method that uses turbidity so turbidity means cloudiness and you can identify that this is a reaction because they're not necessarily going to say to you using turbidity you can identify that it's this sort of reaction because if you look in the symbol equation which they have to give you because this isn't a named example you don't have to know about this reaction but it's just there are basically only two reactions with turbidity that we do in keystage 4 so you're almost certainly going to have done this one um the way you can tell is that you've got that state symbol there so the state symbol means solid and so what that tells you is that you're making a precipitate um you've almost certainly done this reaction so you probably know that it's yellow but again that's not actually in your spec um but so I would look at this reaction i would see that in the products I've got a solid over here i don't have any solids i just have solutions and um and so because that solid is being formed my um my solution is going to go cloudy and what you usually do is you have um a cross on a piece of paper which you can see here and you look at it from above and you see how long does it take until you can't see that cross anymore um this is a perfect opportunity for them to ask you about um ways you could improve this experiment um so you could talk about the fact that um humans are very subjective um humans you know you're looking down you're saying "Oh I think it's gone oh no I think I can still see it." So a better way to monitor this reaction would be to use a light sensor and if you use a light sensor then it can tell you um when a particular amount of um a particular amount of light is coming through or not coming through anymore um someone's asking about writing PPT instead of precipitate so what I would say is that your examiners are not not out to get you your markers are not out to get you um and all of your markers um to mark science of AQA you do have to have been teaching science for a couple of years so it's extremely likely that they would know what you would mean and it's extremely likely that they would say "Yeah I know what you're trying to say." And give you the mark however they don't have to so I would air on the side of caution and write the full word even though I reckon 90 95% of people probably would give you the mark for it because they're not trying to deny you marks if you're writing good chemistry but why would you take the chance um have I been through the procedure yes but you can you should be able to scroll back on the live stream um also if you want full um writtenout methods there was somebody on I think it was the physics live stream um was asking about full writtenout methods i think it was Aisha um so I did make a video for this um this paper which is all the chemistry required practicals paper two there's a link in the description um and that does have for each one um a full written out method like a six mark question that you could just memorize um I have no idea why you're talking about percentage of sodium hydrogen carbonate concentration i actually can't think of a reaction that does use that um yeah so for whatever reaction you're describing you would need to talk about changing the concentration and it's probably worth saying I would change the concentration by diluting it um you might even want to describe how you would do that so sort of saying oh we'd have 2 cm cubed of acid and top it up with eight of water and then 4 cm cubed of acid and top it up with six of water or whatever um that is Yeah that is your Oh right i see what you mean okay so um yes concentration you are supposed to know in moles per dec um biologists quite often use percentages chemists not so much um you use whatever the exam board has told you to use so if they haven't specified stick with moles per dec um but if they say to you I don't know I'm giving you a 10% solution of starch um then you can work with 10% right reversible reactions so in some chemical reactions it's possible for the products to react together to make the original reactants and we call those reversible reactions and you can recognize them because of this here arrow so make sure that you recognize it make sure that you can draw it cuz they have asked before for a little one mark question for you to draw that and it's possible to change the direction of the reversible reaction by changing the reaction conditions you should know that if a reversible reaction is exothermic in one direction then it will be endothermic in the opposite um direction so there is a named example of this and nobody knows this nobody realizes it's there but this one here is in the spec um they could literally say to you what color is hydrated copper sulfate and you are supposed to know that it is blue so if you take some hydrated copper sulfate which is the blue crystals that most of you would have made for the first required practical in um paper one and you heat those up you can make a white powder called anhydroscop sulfate um it's the same thing that some of you might have made by accident in this practical if you heated your crystals for too long um and then if you take that anhydrocop sulfate and you add some water to it then you can turn it back into copper sulfate hydrated copper sulfate the blue one um and if you do that it's an exothermic process so if you take those boring white powder that you've made by accident and you add a little bit of water then the test tube will get hot in your hand because it's an exothermic reaction went too far so when a reversible reaction happens in a closed system so in other words in a closed container no matter can get in or out no energy can get in or out um then eventually it will reach a point called equilibrium and this means that the forward and backward reactions are happening at the same rate or the same speed so both of the reactions are happening at the same time this means that the concentration of the reactants and the products will not change now it's really really important that does not say they are the same as each other because they are not the same as each other don't get confused with your biology diffusion equilibrium um there is an entire multi-billion dollar chemical industry that is totally focused on how to make there be as much product as possible and as little reactant as possible so they are not the same as each other okay so um where we've got that reversible reaction going on um you could be asked say on a graph um to indicate where it's reached equilibrium so I've done a little um arrow here so the point is where my concentrations stop changing this is now basically flat it's a plateau that's the point where um it's reached equilibrium okay so this is our first bit which is um just for higher tier so um it is combined science you need it for combined science but if you're taking the foundation tier papers you don't need this bit so if we've got a system that is at equilibrium and we try to change the reaction conditions so um it's no longer a closed system then the system will respond to counteract the change and this is called the chhatellier's principle so for instance if you add a reactant the system will remove that reactant and if you heat if you heat up the reaction then the system will try to cool it down but there are only two things that it can do to do this it can favor which means speed up the forward reaction or it can favor the backward reaction or the reverse reaction those are the only two things it can do um the difference between equilibrium and dynamic equilibrium is that dynamic is taking into account it's kind of it's making it clearer that both reactions are happening so really all equilibrium is dynamic equilibrium it's just a nicer phrase for it and it's one that we use a lot more at a level um because you're getting that point across that reactants are turning into products and products are turning back into reactants um and yeah uh and the person who asked about the graph no so this is our concentration of the chemical so this would be for the reaction uh A + B turning into C + D so at the start I've got lots of A and B and they get used up and I've got no C and D and I make some and eventually we get to the point where those concentrations are unchanging okay so let's look at some of these so first thing what will happen if I add more hydrochloric acid so I need to have it in my head whatever I try to do the Chita's principle tells me the system is going to shift to counteract that change it's going to be my nemesis it's going to try and undo whatever I've done so it's going to do whatever will remove hydrochloric acid and I've only got two options either this forward reaction is going to happen more or this reverse reaction is going to happen more so if I've added more hydrochloric acid it means to use up the hydrochloric acid which is going to be the forward reaction so I would say the system shifts to remove hydrochloric acid right that's your first easy mark for any kind of equilibrium licitellia principle question you're going to say the system will shift to and then basically undo whatever they said you're going to do so the forward reaction is favored that means it goes faster so the equilibrium shifts to the right so my forward arrow points right therefore equilibrium shifts right so therefore I'm going to make more of this green copper compound okay and you will really commonly find for equilibrium questions that rather than naming compounds or naming chemicals the exam board quite often just say a blue copper compound a pink cobalt compound because they don't want you freaking out because you haven't heard of the chemical before and equilibrium chemistry is quite complicated um so don't be confused if they just don't actually tell you what the chemical is okay so next up we need to talk about pressure so it's important that we understand um what pressure is actually caused by so pressure is caused by particles colliding with the walls of the container they're in so um if I've got twice as many particles like I have on the right hand side then there is twice as high a pressure so if we increase the pressure it will favor the side that has fewer molecules on it so whereas with um adding or taking away a reactant we can just look we can just say oh if I'm adding a reactant it will always push right here we're going to need to look at our simple equation so here you can see I've got three moles of gas on the left and two moles of gas on the right so you can see I'm using those coefficients there right they're important um and I should say pressure is only going to affect um the position of equilibrium if we've got gases cuz if we've got solids they're just going to sit there they're not colliding with anything so first thing if I decrease the pressure the chellier's principle tells me the system will shift to increase the pressure and we increase the pressure by adding particles and the reaction that's going to add particles is this reverse one here so the system shifts to increase the pressure the backward reaction is favored which means it goes faster um because there are more molecules on the left I counted them um so the equilibrium shifts to the left so the yield in other words how much I make of sulfur triioxide is um lower now you might also encounter one where they they could give you a reaction like this and um if they're feeling really mean ask you what happens when you change the pressure if they're not feeling so mean um they could say to you why does changing the pressure not affect the position of this equilibrium and the reason is that if I count up all of those gas particles I've got two over here and I've got two over here so I've got the same number of moles of gas on both sides of the equation therefore changing pressure is going to have no effect okay so then we move on to temperature and this is probably the most confusing one um so we said at the start that if we heat up a reaction at equilibrium the system will shift to try to cool it back down again and the way it does that by is by favoring the endothermic reaction so if I heat this reaction then it's going to favor this one but the reason this is a little bit harder is that very often um they are not going to tell you both bits here so very often what they'll do is not tell you that they'll just tell you the reverse reaction is exothermic so then you need to not just look for the endothermic reaction but also know that if the reverse reaction is exothermic the forward reaction is going to be endothermic so we'd say that the system shifts to reduce the temperature so the forward reaction is favored because it's endothermic so the equilibrium shifts to the right so the mixture turns white um depending on someone's just asked in the comments about could you say something about the yield being higher or lower it depends on the structure of the question so um sometimes they will ask you specifically for an observation in other words something you can see so then you would need to be mentioning um say it's going to be a color change basically um if they just say to you what's the result then yes you absolutely could say the yield would increase the yield would decrease whatever it is and then the last thing is what will happen if we add a catalyst and the answer is absolutely nothing so catalysts speed up the rate of reaction but they speed up the forward reaction and the reverse reaction by the same amount um so if you imagine a catalyst being um like a special chemical that grabs the reactants and puts them together in the right order well in that case they're also in the right order to switch back the other way so adding a catalyst is not going to have any impact on the rate of sorry it's going to have an impact on the rate it's not going to have any impact on the position of equilibrium right quick drink okay um there's a question about saying I don't understand how it shifts left or right so let's see if I can make this clear one more time let's go back to this one because it's probably easiest so um if you're sort of if you're saying that you don't know like you can't get your head around physically what's happening don't worry you don't need to um but basically if you imagine um in each of my reactions here's my blue copper compound and here's my hydrochloric acid right this this is a terrible picture but just roll with it so in order for them to react they have to meet each other they have to collide with each other if I suddenly add a load more hydrochloric acid into that reaction then it's suddenly really likely that this particle here is going to collide with one of them because there's a bunch more of them just hanging around waiting to be collided with um if you're saying you don't get how you're supposed to know which reaction is which um start from the point of view of you're trying to undo whatever has happened so here we've added hydrochloric acid so you're trying to remove it and then look at your two you basically got two reactions in a reversible reaction so here we've got the blue copper compound and hydrochloric acid reacting to make green copper compound and water and we've got the green copper compound and water reacting to make the blue copper compound and hydrochloric acid so if we're trying to get rid of hydrochloric acid it's got to be the reaction that takes hydrochloric acid and turns it into something else hopefully that makes a bit more sense okay where do we get to we got to here right okay so we're on to the second topic which is organic chemistry so this starts off talking all about crude oil so crude oil is an unrefined fossil fuel it's a finite resource so we are using it up faster than it's being produced and one day quite soon it will run out it's found in deposits deep underground trapped in rocks and released by um drilling crude oil was formed from the remains of ancient biomass in other words plankton so millions of years ago as those plankton died and they sank to the bottom of the ocean they were covered in sediment which later hardened to rock and under heat and pressure their bodies transformed into the three fossil fuels coal oil and gas um so the coal is formed from the remains of um plants and phytolankton which are phyto which are plankton that are kind of like plants they're photosynthetic and then the oil and the gas were formed from um the zup plankton from the animal plankton crude oil is a mixture of um molecules called hydrocarbons um mainly of a class called alkanes and hydrocarbons are compounds that contain hydrogen and carbon only with no other elements present so we need to get that only in there so based on that definition of a hydrocarbon you should be able to identify which ones of these are hydrocarbons and explain um why that's the case so if we look at the first one we would say oh no that's not a hydrocarbon because it's got something in it um that isn't hydrogen or carbon uh no you definitely don't need to know the binomial name of plankton um especially not for a chemistry exam my goodness how evil do you think they are second one also not a hydrocarbon because it doesn't have any carbon in it it's not a compound um third one still not a hydrocarbon because it's not a compound we've got carbon we've got hydrogen but they're not bonded together and then lastly yes we do have a hydrocarbon because this is a compound that only contains carbon and hydrogen and of course it's um it's methane okay so um we can split the hydrocarbons into some different types based on their chemistry and methane is the first example um of the alkanes and alkanes are a homologous series so this is a group of chemicals that have similar chemical properties because they have the same chemistry basically so they react in the same way um and the reason for that is that they have the same functional group and the same general formula which we'll get to in a second um so sometimes we describe alkanes as being saturated hydrocarbons um yes this is still foundation so the header is where it says unit 7 organic chemistry if it gets to higher it changes color and it says higher if it doesn't say anything it's foundation um okay so saturated hydrocarbons so the molecule on the left is saturated because every atom is making the maximum number of single bonds to different atoms different is crucial um the molecule on the right is unsaturated oh oh no what just happened ah right hang on i pressed a button come back alanes sorry guys i hit end by accident and it just jumped on right i did all that there we go right sorry about that okay so one on the left is um saturated because every carbon is making the maximum number of bonds one on the right is unsaturated because these two carbon in the middle instead of being bonded to the maximum number of different things they've got one two three bonds oh that was really unhelpful i shouldn't have done that in the same color um one two three bonds and then it's got this double bond to the other carbon um and so we describe it as unsaturated um you need to be able to name and draw at least the first four alkanes so hopefully you're quite familiar with those um and you might have uh encountered I use most elephants prefer bacon to remember their names so most elephants prefer I like the PR for prefer and then bacon and that's my way of um remembering things um you might have lots of other people um lots of other different ways of remembering it whatever works for you frankly um you also need to be able to um give a formula for each one of them and also a general formula so if we look here at the central bit you can see that for every carbon there's a hydrogen above and below so if we call the number of carbons n then the number of hydrogen's going to be 2 n and then we've also got these two hydrogen's that are on the ends so our general formula for alkanes is CN H2N + 2 so we could use that to figure out the formula of any alkan so say we could say oh what if we've got 51 carbons okay so in that case the hydrogen's is going to be two lots of 51 plus two on the end so that'll be 102 so that' be 104 um and you can obviously do that in both um both directions hey some people putting the answer in the channel already good job um okay so we said that crude oil is a mixture important they like putting that one in in as little one marker little multiple choice um a mixture of hydrocarbons um but all of those different compounds have different uses so we've got fuels which are chemicals that are burnt in oxygen to release energy and then feed stocks which the raw materials used in industrial processes to make solvents and lubricants and detergents and polymers but they all have different properties and we can't use them for any of those things if we don't separate them out to just have um a fraction that just has things with the same property in so that brings us on to fractional distillation so this is a process that we can use to split up all of those different um compounds into not pure molecules but fractions so a fraction is a group of compounds that have similar boiling points and when we're talking about mainly a mixture of alkanes that's going to mean molecules that have similar chain lengths um so things like petrol has got um chain lengths maybe sort of from about hexane up to decane um but something like diesel would have much much um longer um carbon chains in there oh okay so when you when you put your um your fractions into this fractionating column we're going to start off we're going to heat them up um to about 350° C now it's really important you're heating them you're not burning them because ultimately the aim of this is that you're um you're going to produce say some petrol that someone else can then put in their car and then they're going to burn it so you cannot start off by burning it you can start off by heating it though so we're going to heat that off when the crude oil goes in and um the vast majority of hydrocarbons are going to evaporate and they're going to rise up that column you will have um some bitin or some residue at the bottom because it's got such a high boiling point it stays as a liquid and that's what can be used to go on and make sort of tarmac and things so up our fractionating column we've got a temperature gradient um or you could just say it's hot at the bottom and cold at the top but obviously if you can remember the phrase temperature gradient saves you a little bit of time so as these um hydrocarbons move up um to where it's much much cooler they're going to start cooling down and condensing and because they have um different boiling points they're going to condense in different places um so we know from uh from paper one that um where you have um small molecules they are held together by weak intermolecular forces and that's what's being overcome when a small molecule um or a small molecular substance I should say boils right we're not breaking any coalent bonds never ever um but because these um because these molecules have different sizes they have weaker or stronger weak intermolecular forces so the boiling point of methane is aboutus 162 cuz it's got very very weak weak intermolecular forces whereas butane is still pretty low but it's a lot higher than the methane because it's got four times as many weakened molecular forces um this is not triple and no you don't need to um memorize exact fractions anymore although you are supposed to know a few of the names of them just like petrol and diesel and napa um but not the whole shebang and not anything about what order they're in um okay so um the smaller molecules are going to end up condensing at the top of the column because they have the lowest boiling point so the molecules are going to carry on rising up the column until they hit the point where their boiling point is so maybe if you imagine like this is 300 and this is 250 and this is 200 i am just pulling these numbers out of the air by the way they're not like real numbers but the point is as it goes up it gets cooler and cooler and when each molecule reaches it particular boiling point then it condenses so the ones that are collected at the top of the column are smaller molecules they've got the lowest boiling point they've also got the lowest viscosity so in other words they're not very sticky um golden syrup is very viscous that's what viscosity means and also they're extremely flammable so they are the very best fuels because they're going to burn the most cleanly and that leads us nicely onto combustion so combustion is the proper chemistry name for burning a fuel in oxygen and based on the amount of oxygen that's available that combustion can be described as either complete or incomplete if there's a lot of oxygen available then all of the fuel can be fully oxidized and we call this complete combustion um and it's going to make carbon dioxide and water as the products if there's insufficient oxygen available then some of the fuel may not be fully oxidized um and instead of making carbon dioxide it will make some carbon monoxide or carbon particulates also known as soot um uh note the names of the different fractions definitely not in like there were definitely some there you can get exam questions that are from the old specification where you needed to know but we threw that one out in 2018 so I wouldn't worry about it um okay so you do need to be able to balance combustion equations um hopefully you can all balance equations fine but um combustion ones are more likely to come up than pretty much anything else so even if you can't balance equations it's a bit too late to learn um there is a little trick you can use for just the complete combustion of um alkanes so if we take some propane and say it's going to react with some oxygen and it's reacting completely it will make carbon dioxide and water so what I'm then going to do is I know the law of conservation of mass tells me that the number of atoms um of a particular type can't change so I've got three carbons on the left therefore I must have three carbons on the right and when I'm balancing an equation all I can do is add coefficients at the front that's what this big number is called it's coefficient i can't change the formula of the compound so I couldn't change that H2O to H2O2 because that would be hydro u hydrogen peroxide and that's not what this reaction makes it makes water but what I can do is change the number of waters so three carbons three carbons i've got eight hydrogen so because water has got two hydrogens's in that means I'm going to need four water and then this is the tricky bit i now need to add up all the oxygen I've got on the right and then those 10 oxygen also have to be on the left and I know I've got O2 molecules so I'm going to need five of those now that works absolutely fine as long as I've got um an alkan that has an odd number of carbons if I've got an even number of carbons then I can use the same method but I will end up having to use halves in my coefficients like 4 and 1/2 oxygen or 6 and 1/2 oxygen you can do that it is now allowed at GCSE um but it tends to confuse people so your alternative is if you got something like this butane here which obviously does not have an odd number of carbons in it you can start off by doubling it and then we can still go through the same process so I've now got eight carbon in total on the left so eight carbon dioxides i've got 20 hydrogen in total so 10 waters and then I'm going to count up all ones I've got and I'm going to have 13 oxygen so you can see there if I hadn't doubled that butane then I would end up with um 6 1/2 moles of oxygen which is just a little bit confusing okay so you also need to know about some of the problems with using hydrocarbon fuels so there are various pollutants that are released when fuels are burned remember you can never just say a pollutant is released a gas is released something bad is released you need to be naming things um so first up obviously carbon dioxide um leads to global warming and all the bad stuff about that that we'll talk about a bit more in unit 9 um if you don't have enough oxygen then instead of making carbon dioxide you make carbon monoxide or even carbon particulates so carbon monoxide is a toxic odless gas that stops your red blood cells from working properly um and carbon particulates can cause um global dimming which literally means the sky is getting darker and less light is getting to Earth um if they if they're wanting you to write an equation for propane they would tell you that it needed to be propane but they don't have to tell you what the formula of propane is because unit 7 is also in this paper so you should know what the formula of propane is because you should know that propane is the third alkan and you should know the general formula so you can work out that it's C3 H8 um almost all fossil fuels have got small amounts of um sulfur in them as impurities so when you burn the fossil fuels you also burn the sulfur and you make sulfur dioxide um and sulfur dioxide along with carbon dioxide and nitrous oxides um are going to cause acid rain um those oxides of nitrogen are produced um inside combustion engines mainly um nitrogen is very unreactive but at super high temperatures it can and will react um so um a combustion engine is one of the places that that's going to happen like in a car but also lightning can make um the same thing happen okay so this graph shows the approximate percentages of different fractions present in crude oil and also the approximate percentages of those fractions that we actually need so as you can see there's a greater demand for petrol and diesel than what we can supply with crude oil alone while larger fractions like fuel oil are more available than necessary so cracking is a process that allows this surplus of fuel oil and other large fractions to be converted into smaller more useful molecules um so it's an example of thermal decomposition decomposition means breaking something down and thermal decomposition is breaking it down using heat because conservation of matter is a thing we know that the number and the type of atoms can't change so when we break apart a large alkan like this decane molecule if we make another alkan like this octane molecule um you can see I've got 10 carbons to start with and then eight on the right hand side so I've got two left over and then 22 hydrogens's and 18 on the right hand side so I've got four left over so that gives me C2H4 which is not an alkan it's missing two oxygen um so instead um the products of cracking are going to be one alkan and then one or more alken which is what this small molecule here is so alkanes we've already talked about alkenes are unsaturated hydrocarbons um which means that they contain double bonds so you need to be able to draw these just like you did for the the alkanes so we start off with a carbonarbon chain and then you're going to add in one double bond to that so even if you've got a carbon chain that's 10 carbons long you're only going to have um one double bond in um then you're going to um add your hydrogen's around making sure that each carbon is only making four bonds in total so a double bond counts as two bonds um so this one is ethine so we name our alkenes in similar ways to our alkanes except we use the suffix in instead of ain um so this is one of the words there are very few words in GCC science where your spelling is going to matter um and where it's important that your marker can read your spelling but this is one of them so I quite often write alkan and alken in capital letters to make it very very clear which one I'm talking about um so this first alken is ethine there is no methine because it's not possible for one carbon atom to make a double bond with itself so we've got ethine and then propene and then butine so those are all the prefixes we've used before and then our fourth one is pentene which is nice and easy to remember because it's just pent like a pentagon or a pentagram or something like that and you can see that for all of those there is only one double bond okay it's a really common mistake that you see people putting double bonds for every carbon and that's not right um alkenes are more reactive molecules than alkanes and we can differentiate between the two types of molecules using a test called the bromine water test um bromine water is to detect the presence of unsaturated bonds so since that's the only difference between an alkan and an alken it allows us to work out which one's which so um before it reacts with any other chemicals bromine water is a transparent orange solution and if you mix it with an alkan then no reaction happens um so it stays orange whereas if you mix it with an unsaturated molecule like an alken then it turns colorless this word here colorless is super super super important um you absolutely cannot write clear clear will not get you any marks and it's called out every single year by the examiners because this one is already clear this is clear i can see through it it just happens to be orange um so the word is colorless um okay so um we've said that when we break down a large molecule we're going to make an alkan and an alken you might be given some simple equations to balance so there are two types in one type you're just given one reactant and one product so we can just kind of go right we've got 25 carbons here 10 here so 25 take 10 is 15 and then we got 52 hydrogens's 22 hydrogens's so 52 take 22 is 30 okay so in that case I'm going to have C15 H30 um and then this is the more challenging one where they tell you that we're making some ethine but you need to figure out how much so we're going to do basically the same thing here and say right C25 down to C9 so that's C16 in total but I know this is in the form of ethine so how many ethine molecules are going to contain 16 carbons oh it's going to be eight you could also do the same thing uh with a hydrogen you could say that's decreased by 32 so how many would give me that H4 there and again it would be eight you need to be able to describe the reaction conditions for two different types of cracking so there's catalytic cracking which involves a catalyst um here's my large alkan that I'm going to crack and there's my catalyst so we said that's a chemical that speeds up the rate of reaction without being used of itself um and um we're going to have a heat source because it's a type of thermal decomposition and then those vapors are passed over the hot catalyst um and we're going to collect the gas so that we can then test it and prove whether it's an alkane or an alken um so catalytic cracking we're going to vaporize that alkan with high temperature um and then we're going to pass it over that zeolyte catalyst the second type of cracking is steam cracking which also requires a very high temperature but it uses steam in place of a catalyst and you'll notice that neither of those methods made any mention of high pressure cuz in GCC chemistry we do not talk about the role of pressure in cracking at all and you won't get a mark if you um if you mention it okay more water time okay so the next chunk of stuff is for the triple scientists so if you are taking combined science you've probably got about maybe as much as 10 minutes before there's going to be anything more for you so if you're in need of a snack now is the time to go get your snack on okay triple only content you can tell cuz it's gone green at the top and it says triple chemistry only so one of the things that alkenes are really useful for is making um polymers so you know all about polymers we'll talk more about them in a second however there are some other reactions you need to know about as well so we can react them with hydrogen in order to turn them back into alkanes and you should know that for that you're going to use a nickel catalyst we can also react them with water to make alcohols using an acid catalyst you don't need to know which acid any acid will do and about 300° C and about what's that doing and about 60 atmospheres which is the pressure and also we can burn them um which they do burn but they produce um quite a smoky flame so in other words they're more likely to undergo incomplete combustion and they're just not very good fuels they're not very useful fuels um so we tend not to use them as fuels because there are other things that we could use that would be better okay so our next group our next homologous series um are the alcohols um so these are defined by their functional group which is a hydroxy group or you can just call it an O group it's definitely not a hydroxide group right hydroxide is the name of the ion so that's a a charged particle that can break off and be out in solution which is not what this is um the general formula for alcohols it's going to be CN H2N + 1 O um and you know just like with any other general formula you can use that to work out the molecular formula of a specific alcohol and we're going to name them in a very similar way to how we have the others so the same prefixes but this time our ending is going to be anol like ethanol or propanol so you should be able to draw them all so basically what you're going to do is draw an alkan and then at the very last minute instead of putting your final H on you're going to put O instead so we have methanol ethanol propanol butinol um in addition to being the sort of active ingredient in alcoholic beverages like beer and wine ethanol and other alcohols are used as solvents so they're used to dissolve things and as fuels so you can burn them um and also as some feed stocks there are two main ways that we can make ethanol so um this one is going to work for all alcohols um but this one is just for ethanol um so we can do fermentation so this is um anorobic respiration by yeast so you give them some sugar um you put them somewhere that is warm not super hot if you get above about 40° your yeast will all die um so warm wet and crucially anorobic if the yeast have oxygen they will just do normal aerobic respiration and all your sugar will just be turned into carbon dioxide um whereas if you don't give them enough oxygen then they will do fermentation instead um that produces um alcohol um very cheaply but also very slowly um and also it's going to be very impure so you're then going to need to distill it if you want to use it for anything other than just drinking it whereas um hydrating our alkenes with some steam that is a way that we can produce any alcohol that we want um it's more expensive because we've got to heat it up to a high temperature and make the steam and so on um but also it's going to give us much much purer alcohol so particularly if you want it for sort of for doing some chemistry with or doing like some DNA sequencing in biology then that's the alcohol that you're going to want other things you should know alcohols dissolve in water to make neutral solutions once again this is a hydroxy group or hydroxile group it's not a hydroxide ion so it's not going to affect the pH so this is just a neutral compound um alcohols burned produce carbon dioxide and water and in fact they're really good fuels because they're basically an alkan with some extra oxygen added so they can burn cleanly in slightly less oxygen than an alkan can you can also react them with sodium which will um release some hydrogen um and you can test that with a uh a squeaky pop test um to the people complaining about my drinking water can you hear how horse I'm getting i've been talking consistently for an hour give me a break um also we can oxidize these alcohols either by microbes so just like bacteria and and things in the air um or chemical oxidizing agents so that could be things like um potassium peranganate is my personal favorite um and that is how we make caroxyic acids which are our next um homologous series sorry completely forgot the word for a second there um okay so our first four members are methaninoic acid ethaninoic acid propioninoic acid and bututininoic acid um so named because it's what you find in rancid butter no I am not joking that's literally where the butt in butane and so on comes from um they dissolve in water to form weak acids so in other words they don't fully ionize we should probably say in aquous solution shouldn't we cuz we want to get all the marks if that was to come up um and so they have um higher pHes compared to say hydrochloric acid or nitric acid which are strong acids at the same concentration um they are acids though so they will react with carbonates um to produce carbon dioxide gas bubbles so we could test them with lime water uh we'll talk about that in the next topic um and they also react with alcohols to make esters so esters are sweet smelling volatile substances so volatile means they evaporate easily and so they're very often used as uh perfumes and flavorings um because they're you know good for that um their functional group is this carbon double bonded to one oxygen and then single bonded to another oxygen if you can't remember which way around they go think about the fact that every element has a certain number of bonds it likes to make and so this oxygen here and this oxygen here are going to make the same number of bonds so if you draw it the right way around then I've got two bonds here and two bonds here if I drew it the wrong way around with a single bond up there and then the double bond down here then this one would be making three bonds and this one would only be making one so that's a useful way of um remembering it um so this is called a condensation reaction and in that condensation reaction water is lost so the hydrogen from the caroxilic acid and the hydroxy from the um alcohol are going to join up together and they're going to make um that's going to be water and that water is going to be lost and then we have this this bond forming here and actually I don't know why I've used that one as an example there is only one example of an esther that you are supposed to know the name of and it's not the one I've drawn it's this one ethile ethaninoate okay so then we get on to polymers now obviously you have had polymers already as part of um unit 2 but there is quite a bit of triple content about polymers which comes up in paper 2 in um both in unit 7 and also in unit 10 so you know from unit 2 it's a very long chain of repeating units you know that the word very is really important you know that those repeating units are called monomers and you know that there are strong coalent bonds inside each polymer thread and then weakened molecular forces between them so new stuff for unit 7 um addition polymers and condensation polymers so an addition polymer is made from monomers that have a double bond in the middle and that double bond is going to break and that's going to allow each one of the carbon atoms to form a new bond and so therefore it can bond to a monomer on either side and we get our very very long chain um so um really important that you can recognize these diagrams and also you can draw them so at the start of the reaction the N oh my thing has gone totally where I didn't expect it to at the start of the reaction the N goes at the start at the end of the reaction the N goes at the end that's how you remember them being which way around they are um also when we start off with our um with our monomer we've got our double bond but in our polymer we only have single bonds also in our polymer we have our bonds going out the side and also we've got our brackets right so you should know all of them um and of course you do still need to be able to name polymers from their monomers so if we start off with chloropropene we finish up with polychloropene um then we have condensation polymerization a lot like that um condensation reaction that we just had with our um esester so for a condensation polymerization reaction you need two functional groups now that could either be um two different monomers that have a functional group at both ends so basically you can have two or you can have an alcohol that has an alcohol group at both ends and you can have a caroxilic acid that has a caroxilic acid group at both ends like that or you can have a situation like this um this is an amino acid called glycine and this has got an amine group at one end and a caroxilic acid group at the other end um so as long as we've got two functional groups either one works and so just like we had with making the esther we're going to lose a water molecule and be left with the rest of our polymer so if you're trying to figure out whether something is an addition polymer or a condensation polymer the trick is are all the atoms you started with in the polymer if they are it's an addition polymer if they're not it's condensation polymer um yes you do need to know how to draw the Esther because it is in your spec oh oh no oh I forgot that I draw on this one hold on um just grab those right there we go um so so there's an example of how that condensation polymerization would work okay you've got some named examples of natural polymers um so you should know about DNA it's not just for biology you need to know it has a double helix structure you need to know that it's made from two polymer chains and you need to know that those polymer chains are made from four complimentary nucleotides not four complimentary bases so the bases are just the bit in the middle of the nucleotide that give us the sequence the nucleotide is the whole thing including the sugar phosphate backbone so the monomer for DNA is nucleotides not um not bases um and you obviously know that um adinine bonds with thymine and cytosine bonds with guanine then we've got proteins which are made from amino acids um this picture of glycine that I just drew is in your spec um I can't remember if it says it's glycine or not but it says amino acids like this so practice drawing that one if you're not familiar with it and then starch is a polymer made from glucose okay so next up we've got unit 8 um if you're doing combined science you're back in the room we're ready for you come back and join us okay so for unit 8 you need to know two different definitions of the word pure which is really annoying because you need to know both of them so in your ordinary everyday life if you were to talk about something as being pure maybe some pure orange juice you would mean nothing's been added to it nothing's been changed about it we might describe it as unadulterated um so this very simplified picture here shows you that within some orange juice we've got some water molecules but we've also got some sugar molecules and those sugar molecules came from the orange i haven't added them in so this is pure orange juice even though it's got different things in it and then in contrast to that in chemistry when we describe a substance as pure um we mean it hasn't been um mixed with any other substance so the next part is all about separating mixtures and this can be done with physical processes like heating or cooling or filtering they're going to split up the different parts of the mixture without changing any of the chemicals so this mixture contains nitrogen with its triple bonds the hydrogen and the ammonia which is the NH3 those three molecules have different boiling points and that means that by controlling the temperature of the mixture we can ensure that the ammonia is a liquid while the hydrogen and nitrogen are still gases and that allows us to separate them out uh okay so we've got some different ways that we can separate them um and we mentioned these also in um unit one but they're still relevant here so we've got filtration which is how we separate um an insoluble solid from a liquid by passing it through filter paper u we've got crystallization which is where we gently heat um a solution that contains a soluble solid so that the solvent evaporates and the soluble solid is left behind um we've then got simple distillation and fractional distillation which is separating things um according to their um boiling points and then finally we've got chromatography which is an analytical technique where we separate out mixtures of liquids um based on how attracted they are to the mobile phase and the stationary phase but we'll go through that in quite a lot of detail because that is a required practical um so in addition to separating out mixtures into pure elements and compounds we need to be able to prove that a substance is pure so this is going to come back in unit 10 when we think about the water required practical one way we can do this is by looking at the melting or boiling point because pure substances have a single unchanging melting and boiling point which you can look up in a data book so for instance we know that pure water boils at exactly 100 so if a sample does not boil at 100 then it's not pure um so GCC physics you've met um melting and heating curves um so you know that we sort of have this situation where the temperature rises and then when you get to a state change the temperature stops rising and then the temperature rises a bit more when you get to your next state change the temperature stops rising and you have these really sharp horizontal bits whereas if you've got an impure substance then you get something that's a bit more weird and wonky and um it doesn't flatten out completely just the gradient um gets a bit shallower um so if you're looking at one like that you know that you're thinking about an impure substance next thing a formulation is a mixture that has been designed as a useful product and we make them by mixing the components in carefully measured quantities to ensure that the product has the required properties and there's a whole bunch of um examples that are named in your spec fuels so that's kind of getting back to unit 7 and organic chemistry cleaning agents paints medicines alloys fertilizers and foods so um bit of a reminder about alloys which you did cover in unit 2 but they're still here um we could talk about the fact that in a pure metal that ions are all one size we can't say particles we could say atoms will let you have atoms but we can't say particles because um the electrons are small so even in a pure metal you have particles of different sizes but you don't have ions of different sizes so they're all one size they're all in regular rows and therefore they can slide over each other whereas in an alloy we've got different sizes so the rows are distorted so they can't slide and that makes it less malleable oh there's my sliding atoms okay so chromatography so this is um the required practical is in this bit but there is also some information you need to know that's not to do with the required practical so one way that we can identify whether a substance is pure and which different substances it contains if it's not pure is by using chromatography there are quite a few different types of chromatography but the only one you need to know details of for the GCC is paper chromatography that doesn't mean that they can't say "Oh here we're going to give you a load of information about a type of chromatography you've never heard of," and then ask you to answer questions about it and one thing that I know we have found in in the 2025 exam series both for GCC and Alevel is that this year for some reason we're seeing a lot more novel context AO3 questions um so you know this is an example of one place that they could give you some information and ask you to work things out so all types of chromatography are used to separate out mixtures and they're all used to analyze what's in them so this isn't like fraction distillation where we're just trying to split up the different bits we want to know what's there all types of chromatography regardless of which one it is have a stationary phase which is going to stay still and a mobile phase which is going to move and the results that we get are based on retention so it's based on the attraction to the stationary phase and the attraction to the mobile phase so if a substance is really attracted to the stationary phase it will stay quite low down if it's really attracted to the mobile phase it will move quite far and when we're thinking about paper chromatography this is linked to solubility so if a substance is very soluble it's very attracted to the solvent which is the mobile phase if it's very insoluble then it's very attracted to the stationary phase which is the paper so required practical you should have set up and analyzed a paper chromatogram um usually it's something like examining a few different types of felt pen to see whether they have the same mixture of inks so there are a few key points that you need to know first up we've got this start line here where we're trying to make sure that everything is starting at the same height and it's re Oh goodness why do I keep pressing that button um it's really important that it's in pencil because if it was in pen then the pen would run and we wouldn't know what was our sample and what was the pen from the start line likewise our solvent which is here which was probably water when you did it but it could be other liquids it could be ethanol it could be methanol that needs to be below the start line right you can't have it coming over the start line because if you did then all your samples would just leech out into that liquid they wouldn't be pulled up the paper by the capillary action um also if you're using a volatile solvent so something like um methanol or ethanol then it's really important that we've got a lid on it to stop it from evaporating and then depending on the type of chromatography we're doing we're either going to have some known chemicals which are our standards so we know what these are and then we're going to compare our sample to those and say is it the same as A is it the same as B or we can calculate an RF value and we can do that quantitatively instead so the results that you did when you actually did this experiment probably looked a little bit like this but in real life you're going to you know in your exam you're going to be given something that's a bit more like this so we can examine these and we can draw conclusions that are qualitative or quantitative so qualitative is no numbers right it's just kind of what's the same what's different so we know that um a pure element or a pure compound will produce a single spot in all solvents so the fact that my sample here has not got a single spot it's got three of them that tells me it's not pure but my standards could well be um so the first thing I could say looking at this is that because my sample has got three spots that means it contains three different colors um now one thing I should say is that for paper chromatography that you do in school you're almost always examining colors so it's really likely in the exam that they say to you how many different colors are in the sample your exam paper is printed in black and white and there is always a few people who are going how was I supposed to know how many different colors there are cuz it's in black and white so when they say how many different colors they mean how many spots are there and then this is a comparative technique right it's like fingerprinting so we can't just look at a spot and say "Oh that's arsenic." What we would need is some arsenic as one of our standards that we can then compare it to and say "Oh this is at the same height." So we can say that my sample contains A because we've got um this spot here that is at the same height as a one and it contains B cuz it's got this spot here that is at the same height as B and then this third one well we don't actually know what that is cuz it doesn't match any of our standards but we know there is a third one we just don't know what that third one is um anything else we need to say oh yes so um this one here this is something that is insoluble uh or you could say it's something that is very attracted to the stationary phase and not very attracted to the um uh to the mobile phase okay so then we get to our quantitative conclusion so this is our using our numbers calculating our RF values and this is useful to do because if I'm going to put my standards on every single um katogram I do that's fine if you're just doing it once in school but if you imagine this is happening in industry they're trying to figure out what's going on with forensics or whatever it would get really timeconuming and really costly and really wasteful to put those standards on every time so instead an RF value allows you to just have a number that can tell you does it match this sample or not so the way that we're going to calculate it is the distance moved by the substance um divided by the distance moved by the solvent so you could have a situation where you actually have to um measure this yourself um you know get your rer out and everything um and it's going to give you a value that is between zero and one i have no idea why it says s after it let's just cross it out so if it's zero it's going to be something like this where it hasn't moved at all if it's one it will be something that it has moved as far as the the solvent up here okay so if you get a value that's not between 0 and one then you've done it wrong and because that RF value is caused by the attraction of the substance to the stationary phase and the mobile phase if you change the mobile phase so if you do it with a different solvent you will get a different RF value and so actually if we want to say conclusively that this is A then we need to do this in a few different solvents and say is it always the same height as A does it always give the same RF value as a so when you're doing these measurements you want to measure from the center of the spot um from the start line and then from the solvent front also to the start line and that's what you're comparing um also in this chemical analysis topic we touch on the idea of instrumental methods versus human methods so instrumental methods are accurate in other words they give you the right answer they're rapid so they're very fast and they're sensitive so that means that they can detect tiny tiny amounts of whatever it is you're trying to detect however they tend to be more expensive cuz you're having to buy the instrument buy the machine or the computer or whatever and also they often need skilled technicians to do them um to run them so that's adding another cost involved and also you might not be able to find people who are willing to work for you for that um yes you do need to memorize the formula to find the RF value okay so we're on to some gas tests so first up we've got our oxygen which will relight a glowing splint so here's my splint blow it out to make it glow and then hold it in some gas and as if by magic eventually there we go it relights okay so that demonstrates that the gas that is being produced in that reaction is in fact um oxygen that's being produced and then our second one is our hydrogen squeaky pop test definitely hydrogen and then we've got our chlorine test now you need to be careful particularly if you're doing triple science because chlorine and chloride ions are not the same thing as each other so if we're testing for um chlorine then we need damp litmus paper in other words it needs to be wet and it doesn't matter what color it is i know there are some of the revision guides say it needs to be red or it needs to be blue literally does not matter the reason it works is that when chlorine dissolves it makes bleach and so it will bleach it whatever color it is but it does have to be wet and then our last test is to bubble um a gas through lime water u which is aquous calcium hydroxide and if it goes cloudy then uh that tells us that it's carbon dioxide so you can see the bubbling there and you can see that gradually it's going a bit milky it's going a bit cloudy this was some quite old lime water so it didn't work very well but you get the general vibe um thermos setting and thermos softening is chemistry but not until unit 10 so we'll get there eventually right um unit 10 is the using resources topic um and no the combined scientists don't need to know that bit it's just the triple bit right okay also flame testing um so we know that um lots of different metals if you burn them in a bun and burn a flame they give you a characteristic color i'll just skip over that bit so this here this crimson flame tells me that I had some lithium ions in there or this um green flame here tells me that I had some copper ions here so the process is that you uh clean off your wire and then you dip the loop into some solid salt or into a salt solution and then you hold it in the flame um and you see the color change of the flame um and yeah that allows you to identify um what was in there so oh yeah so I forgot to speed this video on a bit so yeah so that's an example um of the green flame for the copper um so those are the colors of the um ions that you need to know um also you need to be aware that this is not very good at detecting mixtures of ions so um potassium is really really pale lilac it's very hard to see anyway if you have a sample that has some lithium as well as potassium or some copper as well as potassium you just won't see it so that's where we then need to turn to our flame emission spectroscopy so this is another example of an instrumental method which is accurate rapid and sensitive so we put the sample in a flame and rather than you just looking at the light and saying "Oh I think that's crimson colored," it gets passed through a spectroscope and this produces a line spectrum so basically you get lines in different places that correspond to different wavelengths and by comparing those you can identify what metals are present um then we've got our hydroxide tests so this can also be used for identifying cations ions so first up I've got copper and if I add the sodium let's just turn the sound off cuz I forgot there was sound on that one um so you can see here for copper we get a blue precipitate and then um for iron you can see that dark green precipitate there that's going to be iron 2 plus ions uh no combined scientists don't need to know the flame colors that's why it was green at the top this bit is triple only and then iron 3+ is going to be that orangey brown and then you've also got um the white precipitates so these are calcium and magnesium and aluminium and crucially if you keep on adding sodium hydroxide then the aluminium will eventually um redesolve okay you could be asked to write some ionic equations for these so basically look at the charge on the iron and that's how many hydroxides you need so copper 2 plus needs two hydroxides iron 3+ needs three hydroxides and then you're going to make your precipitates like this so the number of hydroxides is the same as the number of hydroxides that you've added and you would put those in brackets and you obviously don't have any charges inside the formula for the final compound then we've got our sulfate test we add hydrochloric acid to break down any carbonates that are in there because they would give us a false positive then we add some berium chloride and we get a white precipitate if we get a positive test so start so we add some berium chloride and my first tube here absolutely nothing happens so there are no sulfate ions there whereas my second tube here you can see that white precipitate forming and obviously I know that looks kind of pale blue but it's because the original solution was blue so you could be asked to write um simple or ionic equations for that so you should know the formula of berium chloride you should know the formula well they would give you the formula of whatever you adding it to so there I was adding it to copper sulfate and then you should know that this berium sulfate that's being made that's that white precipitate is BSO4 and you would work out what the formula of the other compound was um you can identify we talked before about the fact that because it's got um an S in brackets that's my state symbol that tells me that um that that's a precipitate and that's a white precipitate um or we could just write an ionic equation for it as well so for this next test I'm testing for haly so then we've got our halide ion tests so this one we're going to add um nitric acid again to get rid of any carbonates and then add some um silver nitrate and we get three precipitates that are uh white for chloride ions on the left and then um so we got chloride ions there bromid stop moving bromide ions that are cream and iodide ions that are yellow so basically as you go down the table it gets more um strongly colored so they have asked before for you to like make a prediction about oh what color do you think aine would be then and you would probably say something like orange or brown um yeah so that's pretty much all for that so again they could ask you to write um equations for those in each instance you're just making um the the precipitate that you're seeing is a a silver haly um and it's a precipitate so it's got a solid state symbol after it um and if you add fluoride ions you don't see anything at all because silver fluoride is soluble so it doesn't make a precipitate so you can't see it and then we've got our um bubbling gas through lime water which actually to be honest we already looked at um so we gradually get that white precipitate forming um so just to look at this one quickly um the bit you need is this bit down here sorry I've cut this out of another presentation and given you more than you need so aquous calcium hydroxide that is lime water and then when you add the um carbon dioxide you get calcium carbonate and also some water being made speaking of which right onto unit 9 we've got two topics left to go so um the Earth's atmosphere um today starts off um with about 80% nitrogen and about 20% oxygen now when we say modern we're using that term fairly loosely um because for about the last 200 million years the Earth's atmosphere has been pretty much constant if you know more accurate numbers than 80% 20% that's fine but those are the numbers from your spec so in addition to those there are small proportions of other gases there's a tiny tiny amount of carbon dioxide and also variable amounts of water vapor depending on things like how close do you live to the coast or to rivers or lakes or things um and some noble gases so in other words um things like argon it's really important when you're talking about the water vapor particularly when we get to the greenhouse gas bit that you are using the word vapor um because if it's not water vapor it's not a gas so that's important now a word of advice a bit of exam technique it is incredibly likely that you are asked to compare the atmosphere of the Earth at some point to some point and pretty much every year there's a question that is either early versus modern or it's just asking you about the last 200 years and always about half the students don't read the question and start talking about the other one so just make sure you know are they asking you about 4.6 6 billion years ago or asking are they asking you about 200 years ago so early atmosphere we're talking 4.6 billion years ago when the Earth's atmosphere first um formed and there is limited evidence so sometimes they ask you why can't we say exactly what it's like and the answer is there was limited evidence please don't say nobody was there because nobody was there 65 million years ago when the dinosaurs went extinct but we've got a pretty good idea of what happened at that point because we can look at rocks from the time we can see all these aridium deposits we can see ice cores there's all this evidence that we can use so it doesn't matter that there were no humans alive what matters is that we don't have sufficient evidence so over the time as science has moved on we've learned new things we've gathered more evidence and so sometimes our theories have needed to change to accommodate that because what we do as scientists is we do experiments we collect data and we've updated our theories so the best evidence that we've got says that for about the first 1 billion years of the Earth's history the whole surface was covered in volcanoes and as you know volcanoes are spewing out rock that is so hot it's turned into a liquid what we call lava or magma if it's still inside the volcano so if you think about how hot that must have been it makes complete sense that for the first billion years there weren't any oceans on Earth cuz any water that was present would just turn into a gas and it would boil off um so in addition to spewing out molten rock those volcanoes would have been giving out a selection of gases that formed the Earth's early atmosphere and we currently believe the Earth's early atmosphere was most similar to the atmospheres that we see today on Mars and Venus so if you look at Mars and Venus today um they have still got loads and loads and loads of carbon dioxide that's the main thing um the difference is that um they have varying amounts of the sort of the trace gases so like potentially a little bit of nitrogen potentially a little bit of oxygen you don't really need to know the makeup as long as you know they had shed loads of carbon dioxide um and anything more than that you're going to be given data to compare anyway back to life on Earth so um the volcanoes were giving out loads and loads of carbon dioxide and then smaller amounts of water and nitrogen and ammonia and methane so as Earth aged that volcanic activity started to die down and with it the temperature on Earth began to drop and eventually it dropped low enough that all the water vapor that had been in the Earth's atmosphere was able to condense into liquid water and suddenly we had the Earth's first oceans now that's important for two reasons the first reason is that the carbon dioxide that was in the atmosphere was able to dissolve in those oceans forming chemical compounds called carbonates and those carbonates precipitated out which means they became insoluble solids and they were able to drop to the bottom of the ocean um so this is where we get um lots of our sedimentary rocks from that contain um carbonates and the second thing is that 2.7 billion years ago algae started using that dissolved carbon dioxide for photosynthesis now note it says algae it doesn't say plants plants came quite a bit later and it's important that you do understand that so the carbon dioxide dissolving and the algae doing photosynthesis were both responsible for the start of the carbon dioxide levels coming down now speaking of photosynthesis um you need to know both the word equation and the symbol equation remember that for chemistry if you're asked to give one you have to give that one if they ask you for words give words if they ask you for symbols give symbols about a billion years after the algae that's when the plants evolved and eventually they released enough oxygen that um the level got high enough that um animals could evolve at that point because before then we're just thinking um sort of single-sellled organisms and very simple um organisms we didn't have anything that we could really call an animal so the level of carbon dioxide in the atmosphere was also reduced by the production of sedimentary rocks um like limestone is an example in fact I think it's an example that's named in your specs i'm surprised I haven't got it on my slide and also um other organic compounds were turned into fossil fuels so all those plankton all those sea creatures um died and they had made their shells from those carbonate compounds and then after they died they were covered by sediment so covered by mud and by sand and they were exposed to high temperature and high pressure and therefore were turned into fossil fuels so um the animal plankton are what's turned into gas and oil and then the plant-based plankton and the algae and things that's what's turned into coal so again all of these contributed to the levels of carbon dioxide in the atmosphere um decreasing so quite often you get little database questions like this talking about how the the levels of these gases have changed over time um obviously you're not going to have color but you'd have dotted lines or whatever um so we talk about the fact that for the first um chunk of time the nitrogen is rising and that's because of those volcanoes giving out lots of nitrogen then we'd say that carbon dioxide decreases and crucially to start with it's not because of photosynthesis everyone says it's photosynthesis and to begin with it's not to begin with it's the carbon dioxide dissolving in the oceans then at this point that's when photosynthesis starts kicking in so at that point the carbon dioxide is going down and the oxygen is going up okay next up greenhouse gases so the first thing we need to clear up is that greenhouse gases aren't fundamentally a bad thing okay so everyone thinks greenhouse gases are just terrible but if they didn't exist then um we wouldn't uh it wouldn't be possible for life to survive on Earth because it would just be too cold um so it's worth thinking of greenhouse gases as being a bit like a duvet right so if you've got one duvet you're nice and cozy and happy and everything's good if you've got a second duvet it's getting a little bit warm if someone put six duvets on you you'd be sweating and you'd be really uncomfortable and really unhappy so that's what's going on now um the levels of greenhouse gases have increased to a point where it's just not a good thing anymore um you need to know that carbon dioxide and methane and water vapor are all examples again that word vapor is really important because otherwise we're not talking about a gas so the way that this works is that the greenhouse gases form a kind of blanket around the earth and shortwave radiation can get through that blanket and then um it's absorbed by the earth's surface and it's re-reflected with a bit less energy and a bit of a longer wavelength um say as infrared radiation and that longer wavelength radiation cannot get back out and so it sticks around and it just um warms up the atmosphere now those rising temperatures lead to lots of effects this is common content with biology so hopefully you've already revised it um but a few things if you're talking about um melting it needs to be ice caps not icebergs not just ice an ice cap is millions of square kilm of ice whereas an iceberg is maybe the size of your house right so when the ice caps start melting that's going to have a dramatic impact on the Earth's oceans whereas a small bit of ice melting wouldn't have an impact so definitely ice caps they lead to flooding there's also extreme weather events like hurricanes uh crop failure and habit and famine and if you're going to talk about um habitat destruction and extinction we need to be specifying so we're not particularly worried about well we are worried about temperate um habitats but for the sake of GCC science we're not worried about what's going on in England we're worried about what's going on in the Arctic so talking about polar habitat destruction or Arctic animals going extinct okay so um where do humans come into this so in the last 250 years alone human activities have led the level of carbon dioxide to increase by about 50% and a big chunk of that is because of industrialization and the fact that we've burned so many fossil fuels um so that could be cars lorries airplanes factories and a large proportion of the electricity that we use in the UK is also generated from fossil fuel power stations and to make matters worse where in the past a lot of those carbon dioxide emissions were offset by rainforests and other wooded areas we've been undertaking deforestation on a massive scale um either because we want the timber we want the wood or because we want the space and we chop down the trees so that we can then have um farmland there either growing crops or grazing cattle so deforestation is also having a massive impact on carbon dioxide levels and then finally we've also been digging up pete so you can think of Pete as being a bit like a fancy compost um and we use it both as a fuel and as a fertilizer and it's a really good carbon sink it's a place that carbon is locked up um but not if you burn it and let it all out and then methane is also a really serious greenhouse gas even if it gets less headlines and less time in the news and there are three big human activities that are all majorly contributing to methane levels the first one is cattle farming which I'm sure you've heard of because cattle um particularly grain-fed cattle rather than grass grazing cattle um have a lot of methane um in their farts basically and so that's a huge contributor um to global warming the second thing is rice farming so when you grow rice the plants have to be grown with their roots totally underwater in a rice patty and then um bacteria live in their roots and those bacteria anorobically respire and as they do so they release methane which again is going to contribute to global warming and then the last thing is using landfills so using rubbish dumps and um as stuff decays in those that also releases methane um so why are we interested in what people say about this so um why do scientists publish their data why do people believe them so you need to have this idea that there is a huge amount of data and lots of different scientific groups around the world have done the same experiments so what we call peer review and they found that all that data is um reproducible so when one group of scientists do another group of scientists experiments and they see the same thing um and so you know they've been able to verify each other's data and show that there isn't bias and that this is objective fact that the level of these gases is increasing and it's probably our fault and it's probably going to contribute to global climate change so typical exam question might ask you to justify how the increase in the world's population has impacted the greenhouse effect and the trap that a lot of people fall into is saying "Oh there's more people." So they do more respiration they breathe out more carbon dioxide um which is probably true but is not going to get us our marks so we need to stick with what it says in the specification so we want to talk about the fact that um all those people need more energy so they burn more fossil fuels and they need more farmland so they do more deforestation and then in terms of the methane um bigger world population means more food which I guess is kind of linked to more respiration so you're going to have um more beef farming more rice farming and also we're going to have more waste produced and as that decomposes um then we're going to have um more methane being produced as well um okay so um there are still a few skeptics out there even within the scientific community and part of the issue is that people really want us to be able to pin down as an exact prediction for what's going to happen next and when and why and how but we're actually talking about a really complicated system with hundreds of different variables and it's really hard to produce an exact model so what we tend to do instead is to simplify our models but also people might speculate about what they think might happen and there can be some biased opinions and sometimes the media really feeds into this because they say that they're reporting fairly by which they mean having somebody from both sides of the argument so you'll have like a breakfast TV show where there's one scientist who says this and one person who says that and what they don't make clear is that 99.9% of scientists agree that climate change is happening and that it's kind of our fault and actually the other people are um a really really small minority so it's really important that wherever possible we actually look at the scientific data so yes you should know that there is a minority of scientists that say there might be another explanation um but basically they're very few and far between and well let's not say anything about who's paying them but yeah that um okay so you should know that your carbon footprint is the total amount of carbon dioxide and other greenhouse gases that are given out over the full life cycle of a product service or event um so we can reduce that either by directly reducing emissions so in other words um having fewer lorries driving around burning diesel or by reducing the amount of energy that's used um so things like my computer is currently contributing to global warming because it's using electricity and that electricity was generated in a power plant um probably by burning some some oil or something okay we basically covered all this lot um back in um unit 7 um but just be aware that these can also come up in unit 9 questions so you should know where each one of those pollutants comes from and what the issue it's causing is okay time for a water break right we finally made it to unit 10 so sustainable development is development that meets the needs of current generations without compromising the resources for future generations so in other words you using resources in a way that means that there's enough stuff left over that your grandchildren don't have to deal without it um resources are materials that we can put to use and for thousands of years humans have made use of the resources around them to provide us with warmth shelter food and transport um in order to operate sustainably chemists seek to use um limited resources by like a minimal amount um and they also develop ways of disposing of products at the end of their useful life in ways that ensure that materials and stored energy are utilized so um resources can be separated into those that are finite and those that are renewable finite resources are going to run out because we're using them faster than they're being produced um this might be because they take a very long time to form or it could be because they're very hard to make um whereas renewable resources are unlikely to run out because they can be produced faster than we're currently using them so that doesn't mean that the supply is infinite or endless it just means that more of it can be produced quite quickly um we can also split resources into those that are natural and those that are synthetic so natural resources are raw materials that already exist in the world without humans needing to do anything but they can still run out and that's why they need to be managed sustainably that might mean limiting how much people can use so things like um having quotas or providing additional support to ensure that more of this resource keeps being produced by agriculture so farming in some instances natural materials have been replaced with synthetic materials which are manufactured from chemicals um synthetic materials often offer greater durability so that means they last longer um and they don't break down as quickly and often at a lower cost and they can help to protect the limited supplies of natural materials um so um it's very likely in the exams that you get an evaluate question and I know I've mentioned this loads of times especially to the kids I actually teach but as soon as you see the word evaluate you have to be thinking compare and contrast so for instance if they gave you something like this we would think about things um like uh how much water it takes to make the um the different bags how much can be recycled how these things break down du and then at the end we would write a conclusion right the the mark scheme for a question with evaluate as the command word always includes level three a strongly justified conclusion so you need to make sure that you have picked a side and it doesn't really matter which side it is as long as you then pick some data to back it up okay next up um portable water so portable water is water that is safe to drink um so it needs to not contain any pathogens not contain any toxins and have low levels of salts so we don't mind a little bit of mineral in there um but not lots and lots cuz your kidneys just couldn't cope it might have some other things in it like in the UK we add fluoride to about 10% of our drinking water um because it's um good for preventing tooth decay um and also you know we often sterilize water with chlorine so it would have some of that in that um so portable water is not the same thing as pure water cuz as we said in um the start of unit 8 pure water would be just H2O nothing else added to it no chlorine no fluoride ions no salts so portable water and pure water are not the same as each other um so steps to make some drinking water uh first up we need to find a suitable source um so that could be a river a lake an aquifer um sorry this is for making portable water from fresh water so that's water that doesn't have any salt in it then we're going to pass it through some filter beds to remove some insoluble solids um and then um sterilize it to kill the pathogens so those are microorganisms that could cause disease using chlorine UV or ozone sorry I'm just getting distracted by people diverting people to other people's uh lives cheers guys okay so that's our fresh water now if we're then thinking about sea water we sometimes call this saline as we know sea water loads of salt in it and your kidneys cannot cope with removing that much salt so you can't drink sea water so because we sometimes call salt water saline the process of removing that salt is called desalination and there are two different um two different ways that we can do it um so first one is distillation and the second one is reverse osmosis and both of those are very energyintensive um and the reason we care is because that means they cost more money but if AQA ask you why don't we use this a lot the answer is because they're very energy intensive they they usually don't let you have marks for talking about costs of things so distillation first um so we've mentioned distillation already in the context of fractional distillation but essentially um we're going to um have our salty water in a flask we're going to heat it up until it evaporates um and then cool it down using a condenser until it condenses and then we can have our beaker of distilled water at the end and then reverse osmosis is where we're going to push that salty water through a membrane under very very high uh pressure and the salt can stay on one side and the pure water is forced through so you've obviously revised osmosis for biology paper one think of it as the opposite to osmosis so instead of going from where there's a dilute solution to a concentrated solution which is what happens in osmosis we're going from a concentrated solution to a dilute solution and you've still got that partially permeable membrane in between okay and then the the last place that we can get water from is waste water including sewage so sewage is waste water from homes and it contains toilet waste detergent and toilet paper and both sewage and agricultural waste water so that's from farms require taking out organic matter and also harmful microbes and then we have industrial waste water which comes from factories and um to turn that back into um portable water um we might need to remove organic matter so in other words like leaves and stuff um but also potentially harmful chemicals so if we're going to treat some sewage there are four steps you need to know the first one is screening and grit removal so screening is basically filtering but just instead of using a piece of filter paper you're using a metal grid or a metal mesh that um stuff is passing through and that's going to remove big um insoluble objects so that's going to remove things like the toilet paper then we spin it in centriuge in a process called sedimentation and that's going to separate it out into the semiolid sewage sludge and the liquid effluent then the sewage sludge is anorobically digested using bacteria and the effluent is aerobically biologically treated so obviously anorobic is without oxygen aerobic is with oxygen um so again this is a prime opportunity for an evaluate question so again you need to compare and you need to come up with a conclusion so we would talk about things like um the fact that industrial waste water is going to require lots and lots of steps um and for groundwater we would need to do filtering and sterilizing but you would also need to do that for industrial waste water you could talk about the fact that um purifying water from seawater is only one step but it's very energy intensive and then you write a conclusion and like I said before doesn't really matter which one you pick because you could um use the data for um to argue either conclusion okay so this is my least favorite required practical and the reason it's my least favorite is that there isn't just one single practical that you can learn the method for um is kind of a lot of bitty bits and I don't really see how they could ask you to write a method for this one so the practical consists of both um testing to see whether a sample of water is pure and then testing to see what's in it and then purifying it so first thing we know that pure water doesn't contain dissolved solids because it's pure it's got nothing else in it um so if you're checking about that what you do is you take a sample of the water and you take a watch glass like a little glass dish and you weigh that watch glass and you pour the water onto the top of it and then you heat that very very gently over a hot water bath and at the end of that process when it's totally dry you weigh it again and see what mass of solids are left behind now if it's pure water there will be no solids left behind if it's water that's got some minerals in it some salt in it then you will have solids left behind um it's quite a hard process to do because if you heat it too vigorously it will just crack um and if you don't heat it vigorously enough and you leave some water there then you're going to have a higher mass than you should do um so yeah not ideal and the other thing that can happen is because you have to heat it over a water bath because that's a very gentle heat you're then going to end up with a load of condensation on the bottom of it so if you don't dry the bottom before you weigh it you would also overestimate um how much water um how much uh solid was dissolved in the water sorry um second thing is you should know that pure water has a pH of 7 so you could use any indicator you like for that well an indicator that shows you where pH7 is so I suppose universal indicator is your your best bet um so you're expecting it to be green or you could use a pH probe which would tell you a number like seven also we know that pure water boils exactly 100° so um you could heat it up until it boils and take the temperature with a a temperature probe and then if you are doing triple you could also talk about testing for salt by doing the flame test for the sodium ions and the um silver nitrate test for the hal ions if you're doing combined science those are not in your specification you're not supposed to know about them but if you're doing triple then they could become relevant so they could kind of join together this required practical with the ion testing one and then the second part of this required practical is talking about how you would um distill the water in order to make it pure so we've already seen um one setup for it but here's another setup so you would have your flask containing your sample of water and you would heat it over a Bunson burner so that all the water evaporates all the salts left behind and then it would pass through the condenser into the beaker where you could collect it um and yeah there's another example of the same sort of thing so that um piece of apparatus that's labeled a condenser basically you've got um a hollow glass tube that your steam could travel through and then around that is a kind of jacket that's full of cold water and it helps to cool it down so this is the the water purification required practical though you should have tested some water to see if it's pure but there's a whole bunch of different ways you can do that like it's not like the food test required practical where they say test for starch with iodine test for sugar with benedict solution they're just like uh check if the water's pure and it's kind of up to your teacher to say what they're going to do with you okay so then we get on to metal extraction um so just as kind of context for this this is how the price of copper has changed over the last few years or this this was like new and relevant information when I started teaching it's a bit out of date now but you can see there that the price of copper has dramatically increased over the years and the reason for that is that it's becoming more scarce so most metals are extracted from ores which are rocks that have enough metal compound in them to make extracting it financially viable so we can make money from it um copper is quite an unreactive metal so it's less reactive than carbon and therefore you can extract it by reduction with carbon um but if we do that we're still going to need to um purify the product later with electrolysis uh when we're thinking about ores we can split them into highgrade ores which have quite a lot of um copper or whatever the metal is in them and then also lowgrade ores which have less than 1% so a low low grade ore you're not going to make a lot of money from it but basically we are running out of high-grade ores so we've started extracting stuff from low gradede ores and there are two novel methods of getting that metal that um you need to know about so the first one is biolleaching remember B for biolleaching B for bacteria um so we can use this on low-grade oes but mainly we use it for cleaning up mining sites so if you've had um a quarry and they've been mining ore they've been um digging up that ore um and then there's usually little bits of metal left behind that they didn't get so then we can use this biolleaching to get the rest of it so traditional leeching um is chemical based and it uses cyanide which is a really nasty poison and then that's all just left in the environment so um bleaching is obviously a bit cleaner than that so your special bacteria go through and they make a liquid called a leach and that's going to contain the copper or contain the metal um and then you're going to get the copper out of that by either using displacement with something like iron so basically you throw in a more reactive metal and the copper comes out or by using electrolysis um advantages are that it has very low energy costs it doesn't release any harmful gases so it doesn't really send any carbon dioxide and it's got a high extraction efficiency so it's good at getting a lot of um what's there out but the thing that is a bit rubbish is that it's very slow and then we have phyto mining with a P which uses plants instead um so those special plants that are going to grow on the soil and they're gradually going to absorb all of the copper through their roots at the same time they're absorbing water and then we burn the plants and we then need to extract um the copper from the ash where you know when we burn them the copper will have all turned into copper oxide so what we tend to then do is then um add some acid to it so we then get some um copper sulfate solution and some water and we can then go on and do electrolysis which you learn all about in paper one so this is still quite a high um still quite a slow process but also it has higher energy costs because you're having to burn it and it does release carbon dioxide but it's carbon neutral so what that means is that this plant here as it's doing photosynthesis is taking in a load of carbon dioxide and then when we set fire to it we give out the same amount of carbon dioxide so it's not like where you're burning a fossil fuel and you're giving out carbon dioxide that's been locked up for millions of years and wouldn't be in the atmosphere anyway you're just putting it back to the point that it would be in if you hadn't grown the plants in the first place um another advantage of both of these methods is that we're not having to quarry or mine to get the ore out of the ground um so it's not going to destroy as many habitats it's going to be better for the environment all right let's see what the comments are up to um you can do these processes on other kinds of metal but I'll be honest I have never seen an exam question about anything other than copper so I tend to talk about it in terms of um copper okay life cycle assessment oh I skipped a bit no um so life cycle assessment is just a way of analyzing the life of a product to see how much water and energy is used and the effects on the environment at different stages so basically when you decide you're going to develop a project and you go to the bank to ask for a loan they want to know how good is it going to be for the environment so you have to answer a bunch of questions about um about the the different um stages of making the product so the four main stages are extracting and processing the raw materials the manufacturer and packaging the use of the product throughout its lifetime and the disposal at the end and then transport kind of runs through all of those because obviously you have to transport the raw materials you have to transport the packaging you have to transport it to get it to the person who's going to use it and you have to transport it to the rubbish dump at the end of its life so um they they could sort of ask you for particular types of questions that might occur in an LCA which one of those um which one of those categories would it fit into so things like where are the materials found that's about the raw materials um or things about like is it biodegradable can it be broken down by bacteria that's about the disposal so the problem we have with LCA's is that um the person who wants to make the product writes it so therefore they might not be objective they might kind of argue more in favor of it being environmentally friendly even if it's a bit ambiguous and there isn't really a right answer the other thing is that part of it is about the impact on the environment in terms of things like carbon dioxide emissions but if you imagine if you were going to I don't know um manufacture a computer and then that computer needed to be transported um to a warehouse for storage there's no way short of you putting a big balloon on the back of a lor's exhaust pipe which I'm pretty sure wouldn't work anyway for you to actually know how much carbon dioxide that process would release and when someone uses the computer and they use the electricity that has been formed from burning fossil fuels actually you don't know how many hours a day are they going to run that computer for um and even if you did you don't know for any particular day what proportion of the UK's electricity is going to come from burning fossil fuels it's like if it's a very sunny day we make more use of solar panels because we can so it gets it's really hard to actually pin down an exact value for some of these things so again evaluate type questions that come up as an LCA make sure you're using the data in the table make sure you're actually doing something with it so you can't just state oh glass has a density of 2.5 polycarbonate has a density of 1.3 that's not enough we want to actually do some mathematical operations so say things like the glass is nearly twice as dense and then also think about how it relates back to these five areas so the fact that it's that the glass is very dense means that it's going to be heavier and that means that when it comes to transporting it um the lorry is going to spend more fuel so that's going to be worse for the environment because it's going to release more carbon dioxide so yeah same sort of thing and again make sure you write a conclusion okay uh then we get to our disposal bit so um reducing means sort of cutting down and not um buying more stuff than you need so things like when you go to the supermarket not buying individually wrapped vegetables but just sort of having them all um in your basket without wrapping them um reusing would be things like when you go to the supermarket taking a bag for life with you and just using it over and over and then recycling is specifically when we're processing something to make a new product so you know melting it down or tearing it up or mixing it with other things there's got to be some processing involved now there are lots of good reasons for us to revise there's the fact that it's going to save us some money because raw materials are scarce and expensive and it might be cheaper to recycle them than to extract you now that's not always true so things like aluminium is really expensive to make from scratch it's a lot cheaper to recycle so that's why we are really good at recycling cans whereas plastic it costs about the same so that's why even if you as a human at home put your plastic into the recycling bin there's a very good chance that it doesn't actually get recycled because the difference in cost doesn't really make it worthwhile recycling is also good for the environment because it has lower energy costs and it requires less mining so that's going to disrupt less habitats and it requires less landfill space because waste is being recycled rather than just put in landfill and also it's just socially a good thing to do so recycling is more sustainable that idea about us um leaving resources that are available for the generations that come after us um yeah let me give you a um someone else is going to have to leave a comment because I can never read the very last comment that's been left so I know that Mtorio has left a comment but I don't actually know what it is but I'll come to it once someone comments something else okay okay so you need to be able to discuss the recycling of aluminium and glass and paper um so when metal's recycled it's often just all stuck in a box together so the first thing that we need to do is to remove any iron or steel which we can do with magnets and then we get the aluminium cans and we squash them and shred them into little pieces blast them with hot air to remove any labels or ink and then we feed them into a furnace at about 750° C which is hot enough to melt the aluminium and so any impurities in there which we call dross rise up to the surface and they can be scraped off and the pure aluminium cools down and is cast into metal blocks called ingots which are then sent off to a rolling mill where they can be flattened out into thin sheets for making new cans density question about copper h it'll be something to do with comparing it to another material so I can't I can't think of a good one but maybe something like if you were going to make a a tennis racket out of carbon fiber versus copper i don't know why you would make a tennis racket out of copper but let's just go with it um so if the copper is denser that would make it heavier and so that would make it harder to use it's usually either to do with how difficult it is to transport or how difficult it is to use okay so then we get on to our glass um so again we need to separate it out from sort of paper and cans and other stuff that's recycled at the same time uh then we use lasers to color sort it we crush it up to make teeny tiny bits of glass that we call color it melt it in a furnace at about 1500° C divide it up into gobs which is sort of blobs of melted glass and then blow it out into new bottles and jars and then recycling paper and cardboard is often quite a lot harder than glass and aluminium uh for two reasons so one is there are so many different types of paper and card and they all have to be recycled in different ways um and it's a lot harder to sort those out so whereas you can use a laser to sort the different colors of glass into different colors um you kind of have to use a human to go through your paper waste and say "Oh this is copy paper and this is cardboard and everything." Um and then the other thing is that whereas you can just keep recycling aluminium over and over and over and it's still good with paper each time you recycle it the fibers get shorter and the paper gets weaker and so actually you can't really have just recycled paper they sort of tend to even recycled paper is made out of 50% recycled paper and 50% virgin fibers like brand new um paper because otherwise it's just it just falls apart basically um yeah so basically we separate it all out shred it up give it a clean um screen it so again pass it through that mesh grid um to remove any paper clips or staples or anything then add in some new virgin fibers and press it to form new paper so um they quite like to give you sort of like suggest questions about why it's difficult to recycle so things like milk bottles with green lids it's really hard to recycle them because if you just melt down the milk bottle you get slightly greenish plastic and nobody wants to buy it um and copper even though it's worth loads of money there isn't a way to easily separate it from other metals so again you're relying on humans to go through and handpick it and that makes it really expensive um so yeah we've sort of said um you could be asked about why it's good to recycle so we could talk about resources being limited and space at the landfill site being limited and also the fact that um metal is going to um have fatigue over time so if you just um just try and reuse something it could actually be dangerous depending on what you're using it for okay we're now on to triple science stuff again unscheduled water break okay so there are um when we talk about corrosion we're talking about substances being destroyed because they're reacting with chemicals in the environment usually oxygen and water but it could be other things as well um so rusting is one example of corrosion but there are two important things to bear in mind because rusting only applies to a very narrow window it has to be iron um so these copper nails have not rusted because they're not made of iron they're made of copper so we say they're corroded instead and the second thing is that when something rusts when an iron object rusts it reacts with oxygen and water so there are a number of different ways that we can stop an object from corroding or from rusting um and they mainly revolve around stopping oxygen or water from getting to the metal so in the top right here I've got an iron nail and because it's not currently protected over time it will rust so look it's gone orange um if I use a barrier method then that would stop oxygen from getting to it so that could be greasing a bike chain painting some railings electroplating cheap earrings you know like when you buy CLA's accessories earrings and they turn your ears green after a little while it's because they've got a very thin layer of silver over the outside of some copper and once the silver has kind of worn away and the copper can get to your ears then it starts oxidizing and that's why it goes green so um a barrier method will stop um stop the metal from tarnishing and corroding so you can see my my second nail here has got this uh layer of red paint around it and therefore it doesn't rust but the thing with barrier methods is they only work for as long as the barrier remains intact so if you get a scratch in the paint like my third nail here that one can still rust because now the um the oxygen can get to it um also some metals form their own natural barrier and aluminium is one example of this so um aluminium reacts very quickly with oxygen to make aluminium oxide and aluminium oxide is very resistant to corrosion so then no more oxygen can get to the aluminium because it can't get through the aluminium oxide um then we've got galvanizing which is a little bit different so this is where we use a more reactive metal um to we call it sacrificial protection basically the oxygen reacts with the more reactive metal or the water reacts with the more reactive metal instead of the metal we're trying to protect so they do it sometimes with ships where they have a big block of magnesium with them that they're kind of dragging through the water and the water reacts with the magnesium instead of with the ship um but again this kind of can run out so um this here is my incinerator which was galvanized so it had zinc in with the iron and the oxygen would react with the zinc instead but eventually all of the zinc had oxidized and so the oxygen then started reacting with iron and eventually it just went really rusty so this is a really classic um experiment that's come up quite a few times so we've got a bunch of different um nails in different tubes and we're describing what would happen to each one of them and so the point is that in order for it to rust it has to have access to oxygen and water so first one will rust it has oxygen and it has water second one won't rust cuz water can't get in third one is covered in paint so it won't rust cuz oxygen can't get in and water to be fair fourth one is covered in paint but it's got um what you call it oh words dear what are you doing um the paint has flaked off at one point it's got a scratch in it um so the oxygen can get in so it will rust um the fifth one um has been galvanized so um that's why it's darker and why it says number five galvanized nail so it won't rust cuz the oxygen will react with the zinc instead the sixth one still won't rust because as long as the zinc is anywhere on there it will um it will be oxidized preferentially so unlike the paint it doesn't matter if it gets scratched and then the last one won't rust cuz oxygen can't get in um okay so we also we met alloys in unit 2 and we talked about all that lot and we also talked about it briefly in unit 8 um you need to be aware that alloys are an example of a formulation so we talked about this in unit 8 as well um but then the triple bit special is that we have seven named alloys so bronze is an alloy of copper with a small amount of tin and that was what we used to use to make two p two pence pieces up until 1992 when uh copper got too expensive then the second alloy is brass which is used to make the pins of electrical plugs um it's got a lot of copper and a little bit amount of zinc to harden it so that the plug doesn't bend when you stick it in the wall some jewelry is made out of pure gold but the majority is made of gold alloys where the gold's been mixed with other metals to make it less soft so it's less likely to get dented or scratched or damaged and also alloying gold allows us to have different colors of gold like white gold or rose gold we assess the purity of gold using a carrot system so 24 karat gold is pure gold and then anything else you can work out as a percentage based on that so 18 karat gold is 75% gold and 25% will be other metals like silver and copper and zinc um then steels are a group of alloys based on iron and they contain specific amounts of either carbon or other metals um so high carbon steel as the name would suggest has quite a lot of carbon in it and that makes it very hard and very strong but also quite brittle so if you push it from the side um it could fracture and it could break so we tend to use it for things that need a high amount of force but it's always in a predictable direction so things like cutting tools drills where it's just going up and down in a factory and it's never having sort of an unexpected um force acting on it whereas lowcarbon steels are softer so they're more easily shaped so we can use them for things like making car bodies stainless steels um resist corrosion so they're useful for making cutlery um because lots of our foods have acids in that would um eat away and tarnish um just pure iron cutlery and then you also need to know that aluminium alloys um are very low density if you think about where aluminium is on the periodic table it's really near the top it's um got a much lower mass compared to um things like iron and so that makes it really useful for things like making aircraft um okay so then ceramics are non- metallic solids that are usually made from a raw material that is heated to a very high temperature and there are two types that you need to know about clay ceramics like pottery and glass and the key difference is that clay ceramics are made in a furnace from clay and they're opaque whereas glass ceramics are transparent and that will affect what you're going to use them for um you know whether it's important to be able to see through something or not um they're both hard and they're waterproof but they're brittle and they shatter easily if you drop them you could be asked about the use of ceramics and what properties they have that makes them appropriate for that use so for instance we said they're waterproof so therefore um they're useful as bathroom tiles and it doesn't really matter that they're quite fragile because they're you know glued onto the wall um then the glass ceramics we can split into two types there's soda lime glass which is used for everyday uses like making windows and tumblers for drinking out of and it's made from um sand sodium carbonate um and that's the soda bit and limestone so that's the lime bit hence soda lime um but it doesn't have a very high melting point and that could be problematic if you want to use it for something that's going to reach a very high temperature so like lab use um so for things that are going to get hot whether that's using them in a chemistry lab or using them in a kitchen um you use borosyicate glass and that's made from sand so that's the silica bit and boron triioxide which is where the boro path name comes from um again we've mentioned polymers but we did say that there was some more new stuff that's in unit uh 10 so polymers can have different properties like different melting points and the properties will differ partly based on the monomer so if you're making it out of a different chemical it's not going to be the same it's not going to have the same properties but also the reaction conditions which include um the temperature the pressure and the catalyst ceramics are poor conductors of electricity because they haven't got any charged particles okay um so classic example of polymers with different properties low density polyethine and high density polyethine so low density is made under super super high pressure pretty high temperature little bit of oxygen in there as a catalyst and when the polymer forms it makes branches and those branches stop the chains from packing too closely together whereas um highdensity polyethine is made under relatively low pressure so I mean it's still 10 times higher than the pressure in the room right now but compared to a thousand for the LDP slightly lower temperature and a catalyst called a ziglanata catalyst and when it forms the polymer chains get into these really regular rows that pack tightly together and so the final polymer is much harder and also much denser then totally separate from that you also need to know about thermosetting and thermosoftening polymers so if you look at a thermosetting polymer on a molecular level it looks kind of like a wonky brick wall they have cross links between the chains and those cross links are covealent bonds so they need a huge amount of energy to break and that just doesn't happen and so that means that your thermos setting polymer if you heat it up it doesn't melt it just eventually char whereas thermosoftening polymers don't have cross links they don't have coalent bonds attaching one polymer chain to the next one and so that means they do melt um when the temperature gets high so thermosoftening polymers are absolutely no good for anything heat sensitive like if you had a saucepan and you want to make a handle for the saucepan there's no good making it out of a thermosoftening polymer cuz it would just melt off then we have our composites so you make a composite material by combining two or more raw materials but like not mushing them all up totally together you're still going to distinctly have areas of each material um and we call the two parts the support so that's the bit that is um like providing the structure and then the matrix or the binder um which is what's going to kind of hold the whole thing together um so the um uh we've got a couple of different examples that we need to talk about and the point with these examples is that um the composite material has some properties from um from both of the bits that make it up we've got some natural examples and some synthetic examples so wood is an example of a natural polymer it's got that soft cellulose that's found in all plant cell walls but it's combined with lignen which is harder so the lignon fibers form um the reinforcement or the support um and they're surrounded by the cellulose matrix um then examples of um synthetic ones so steel reinforced concrete has concrete which has really good compressive strength so it it resists being squashed um but if you try and um bend it it's fragile and it shatters so the steel reinforcement has really good tensile strength which means you can stretch it and bend it and so it's able to resist that so steel reinforced concrete is even stronger and better than just regular concrete because it resists being squashed and it resists being squashed stretched even um plywood is another example so where you've got very thin layers of wood held together with glue so the plywood is the reinforcement um and the glue is the matrix okay the harbor process is an industrial chemical process that was developed by Fritz Harour in the early part of the 20th century as a way to make ammonia which is the main component of most of the fertilizers that our farmers use on their crops to make sure that their yields are high enough that they can grow enough food to feed everybody um and Harvard actually won the Nobel Prize for his work to develop this process at the same time you can use ammonia for some slightly less nice things like making explosives and chemical weapons um ammonia is one of the eight named examples of small coalent molecules that you drew in unit two and this is a quick reminder that stuff from unit 1 2 and three can come up in paper two as well not for a big question um but for little one or two markers those fundamental topics it does say in the front of your spec they can be examined in either paper so just bear that in mind um so when ammonia boils it's not the strong coalent bonds inside the molecule that break it's the weak intermolecular forces between the molecules so ammonia is a gas at room temperature it's made out of hydrogen and nitrogen which are also gases reacting in a 3:1 ratio which we can see in the equation there um and it's a reversible reaction which you can tell from the funky arrow um to make the ammonia um the nitrogen that we're going to use is extracted from the air but it does need to be separated from the other gases because if there was oxygen in there as well the oxygen would just react with everything and ruin it all and then the hydrogen can be made in a few different ways but most commonly by reacting um methane with steam so for the reaction to take place the mixture of gases in that 3:1 ratio are fed into the reactor and there they react together in the presence of a hot iron catalyst at a temperature of 450° C and a pressure of about 200 atmospheric pressures the iron catalyst is used because it helps to speed up the rate of reaction without needing to heat things up further um and that's going to help reduce energy costs and be better for the environment the other two conditions are both compromises and this links back to unit six and you do need to be able to talk about them so in unit six you learned that a reversible reaction can reach equilibrium where the two reactions are happening at the same rate and which principle tells us that if we make a change the system will shift to counteract that change so if we increase the pressure the system will try to decrease it if I look at my equation for the harbor process I can see that on the left hand side I've got five moles of gases and on the right hand side I've got two moles of gases so therefore if I increase the pressure to decrease the pressure I'm going to favor the forward reaction so high pressure is also going to give me more yield but high pressure is dangerous because it's more likely to explode try and give as much detail as you can right don't just say "Oh it's dangerous." Say why and also it's more expensive because you're going to need equipment that can cope with being at 200 atmospheres so we pick a pressure that is at basically as high as we can afford to if we pushed it higher we could have a bigger yield but it would be more dangerous and more expensive and then temperature um is a bit more of a nightmare so um we said that in a reversible reaction one reaction is always exothermic and one reaction is endothermic so this reaction the forward reaction is exothermic so the forward reaction gets hotter so if I increase the temperature that will favor the endothermic reaction which gets colder which here is the backwards reaction so in other words it's going to make that happen so I'm going to make less ammonia if it gets hotter so then you would think well why not do it at a very very cold temperature because then we'll get lots of ammonia right so you can sort of you can see here on the graph um that as as the temperature gets hotter the amount of ammonia is going down right so you would think well why not use a cold temperature but the problem is that cold reactions go very slowly they have a slow rate so we would make more ammonia but it would take us all year so again the temperature is a compromise and if I use about 450° that means I can make about 17% ammonia which is about enough to make it economically viable now because only 17% of that nitrogen and hydrogen are reacting what comes out of the reactor is a mixture of gases so we put them into this condenser here so this is very very cool and we cool the mixture so that it's cold enough that um that the ammonia has become um a liquid so therefore it has to be uh below minus 34 or I suppose belowus 33 which is minus 34 i'm confusing myself now um but it needs to be warm enough that nitrogen is still um still a gas so it's going to be in that gap there and then um the ammonia is going to be removed and the leftover hydrogen and nitrogen is just going to be recycled back into the reactor there so we said that one of the biggest uses for ammonia is um in fertilizers so you should know about NPK fertilizers so those three are the um the chemical symbols for nitrogen and phosphorus and potassium which are all things that plants need and if they don't have enough of them that would reduce the yield but if we do add um fertilizers to our crops they cost a lot of money it costs a long time to spread them on and also if it rains then the fertilizer can run off into the waterways so run off into nearby um lakes and rivers and when that happens it can trigger an algal bloom where all the little green algae go oo tasty tasty fertilizer and they grow like mad and then when they run out of nutrients and they die off bacteria come and break them down and in doing so they use up all the oxygen in the water and they deoxxygenate the water and then everything dies um so MPK fertilizers are an example of synthetic fertilizers so they're made from chemicals um and they're formulations so we know exactly um what proportion all of the different compounds are and they contain salts of nitrogen phosphorus and potassium so they're not going to contain just those pure elements um so we add nitrogen in the form of ammonia um we can mine potassium chloride potassium sulfate and phosphate rock um the potassium chloride and the potassium sulfate are soluble so we can just put those in the fertilizer but a phosphate rock is a rock so you can't put it in um so what you do is you break down that rock by using nitric acid or less commonly sulfuric acid and that produces phosphoric acid and then calcium nitrate or calcium sulfate um which is you know what we're going to um be left with um so again you could get some sort of suggesty evaluate type questions uh where they ask you about um different fertilizers and and whether a farmer would need to use one or both of them and why he might choose a certain way so here we could say that um making ammonium phosphate takes more raw ingredients because you've got the sulfuric acid and the phosphate rock and the ammonia and there's more processes involved but also you're going to need both of those chemicals because um the crops do need nitrogen and phosphorus and potassium and if you don't have the potassium chloride then you don't have any potassium in there so that's no good at all um this is also a really good example of where um they can throw in some more math skills so you know that your maths marks are 20% of the paper so things like just asking you to work out a simple percentage um and if they're feeling mean they could also ask you to convert so putting it into grams um also doing similar sort of things with ratios so working that sort of thing out and that is us done for content so a couple of last little reminders before we call it a day um if you haven't been already please write in black pen your paper is going to be scanned if it's not in black it makes it really hard to read it make sure you stay inside the box because they're going to chop off the margins so if you write outside the box your marker might not get to see what you've written um watch out for sneaky units um less relevant for this paper to be fair if you're writing an extended response question bullet points are not just allowed they are encouraged they make your markers life easier um I'm sure I've emphasized this enough tonight but just in case for the final time if you have an evaluate question you must compare the data and you must write a conclusion if you do not write a conclusion you cannot have the last two marks if you do write a method particularly for those unit six um practicals make sure that you've identified your variables and that you've written a method that would work so it will actually allow you to answer the question so your variables remember mix you modify your independent variable if you're drawing a graph it goes on the x-axis your dependent variable is the one you record and that goes on the y- axis um control variables are to make it a valid experiment if you do the same experiment and you get the same results when you do it again with the same method that is repeatable whereas if somebody else does some peer review and they get the same pattern of results that's reproducible um resolution is the smallest measurement that your equipment can measure the difference between so that could be for something like a temperature or for a measuring cylinder um computerized instrumental methods are accurate rapid and sensitive and if you're drawing any graphs your line of best fit doesn't have to be straight um systematic errors are caused by your equipment being poorly calibrated so where it's always out by the same amount and you can just correct by say subtracting half a kilogram each time whereas random errors are caused by your equipment not being precise enough and by natural fluctuation in the value so in order to do that you would repeat you would remove any anomalous data any data that doesn't fit the pattern and then you would calculate a mean and then last but not least um mathematical requirements so we've mentioned these are going to make up 20% of all the marks in the paper um so decimals I'm sure we're all fine with standard form make sure you know how to use your calculator make sure you know where your times 10 to the whatever button is uh fractions ratios percentages estimates significant figures um watch out for those because they're always worth mark um calculating means I haven't seen any medians or modes in ages but they are listed in there uh frequency tables bar charts you know if you're drawing bar charts make sure you're leaving spaces between the bars make sure you're using your ruler which I'm currently not um histograms scatter diagrams um orders of magnitude these symbols here if you haven't seen them already I'm sure you know uh less than but if you've got a double less than it just means a lot smaller than uh the fish alpha thing is directly proportional and the little wiggly line is a tild and that means approximately uh changing subjects of the equation probably not so relevant for chemistry paper 2 likewise substituting and solving equations and then converting between graphs and tables is just can you take a table of data and uh plot it on a graph and that's the whole lot so uh yeah good luck and thanks for all the fish um I'll hang around for a little while ask if anyone does have any other questions or bits they want me to go back over um that I can do um but if not um good luck for the exam on um Friday I'll just pop it back to the screen I showed right at the start with all the other uh resources just in case any of these are useful to you um so if you weren't with us at the start uh this is the pre-recorded one which is only about 70 minutes long um these are questions if you want to check that you've memorized all the um facts from the spec and these are the answer videos where I go through them this is a new video from this week about the required practicals so there's a method for each one of them and also just talking through them and then these are individual unit summaries for each unit so if you say if you joined us after the first unit and you want to go just revise um unit six you can find that playlist there all of those links are in the description and if not then uh thanks for coming along i would just say that um carbon monoxide is toxic cuz that's all the spec says chemical analysis is quite a big topic so if you are um if you're wanting to do the whole topic what I would do is go to this address here um it is case sensitive so you need to have the capitals in the right place or there's a link in the description that you can click on um and then watch the chemical analysis video which is about 15 20 minutes um what do I mean by compromise so compromise means um like if you think about the um the temperature uh for the harbor process if you have a really low temperature you'll make lots of ammonia but you'll do it really slowly if you have a really high temperature you'll make hardly any ammonia but you'll do it really quickly and we want to make lots of ammonia and do it fast so instead we pick a temperature in between where we make quite a bit of ammonia and we make it quite fast okay so it looks like um everyone's pretty happy so um good luck with the revision folks and uh yeah see you soon