this is a revision video for a QA GCC combined science chemistry paper 1 it covers all the key information but not in a huge amount of detail because otherwise it would be about 40 hours long the idea is that you can use this video to give you an overview of everything on the paper and also that you can use it as a last minute cram right before the exam so the first part the GCSE is all about elements compounds and mixtures and to understand that you need to understand a little bit about assets all substances are made of atoms and an atom is the smallest part of an element that can exist and still be that element there are about a hundred different elements and they are shown in the periodic table compounds are formed when those elements combine together in chemical reactions compounds contain two or more elements chemically combined in fixed proportions now chemically combined is just your example its way of saying bonded and in fixed proportions means that you're always going to have the same ratio of those elements and that's what a formula tells us now you do need to be able to name those compounds from their formula so there are two rules that you need to know say you've got two elements in a compound and if we're talking about ionic compounds we're gonna have a metal and a nonmetal firstly you take the name of the metal and you just leave it as it is then you take the first syllable of the name of the nonmetal so say it's oxygen you take ox it it's chlorine you take claw if it's fluorine you take floor and you then put our need on the end of this so to give you some examples if we have iron and oxygen we take the name in the metal that's iron we take the first syllable of oxygen ox now we put ID on the end iron oxide copper first syllable of chlorine is claw per ID on the end copper chloride now the second rule is that if you have three elements and one of them is oxygen so you start off exactly the same you add the first syllable of the nonmetal and then you put 8 on the end to signify that there's oxygen involved so if we've got copper sulfur and oxygen now got copper sulfate if we've got silver nitrogen and oxygen we've got silver nitrate and if we've got aluminium phosphorus and oxygen we've got aluminium phosphate now the next bit of GCSE is about separating mixtures so firstly we should probably define a mixture so a mixture is two or more elements or compounds that are not chemically combined together they're not bonded so for instance this little picture here has got some nitrogen you can see it there with its triple covalent bond it's got some hydrogen and it's also got some ammonia nh3 now they can be separated from each other by physical processes so what we mean by physical process is something like heating or cooling or filtering well we're physically moving the molecules around but there aren't any chemical reactions happening so we're not changing the chemicals we've got at the end of this separation process I'm still don't have nitrogen and hydrogen and ammonia they're just going to have been split up from each other now there are five physical separation processes that you need to know about some of them come back a little bit later in the GCC in slightly more detail but just to run through them here we start off and we've got filtration and I'm going to use filtration if I've got an insoluble solid so that's a solid that won't dissolve say sand would be a classic example and it's mixed up with a liquid and I want to separate that insoluble solid from the liquid so I've got a funnel I've got some filter paper and I'm gonna put the mixture through that and my insoluble solid is going to stay on the filter paper we call that the residue and the liquid is going to go through the filter paper and we call that the filtrate my next method of separation is crystallization so this is a way of separating a soluble solid from a liquid so this is how you would get salt back out of salty water for instance or how you would make crystals or something like a copper sulfate which is this lovely blue color so the only piece of equipment I need is an evaporating Basin and then I need some kind of heat source so that could be using a Bunsen burner it could be using a water bath you might have even done this just by leaving it on the side in the sunshine and the liquid is going to evaporate off and it's going to leave us the solid behind so if you have a method question which asks you to describe how you would obtain a pure dry sample of a salt this is the technique you want to be talking about oh I'd make my copper sulfate and then I put it in evaporating racing and I would use a heat source to remove all the liquid so I would just be left with my dry salt at the end of it then we've got simple distillation and a fractional distillation where we're going to use heating and cooling to separate mixtures of liquid and finally we've got chromatography which is also going to separate out some liquids but this is going to be so that we can analyze them or rather so we can use the patter words okay so fractional distillation is going to come back in more detail in paper - when you're looking at your organic chemistry but we can talk about it quickly now their only difference between fractional distillation and simple distillation is that instead of just getting the one sample that I'm trying to get out I can have a whole range of different liquids that are all mixed together and I can get all of them so you might recognize this picture as a fractionating column that we would use to extract the different fractions from crude oil and it's going to separate liquids according to their boiling point so according to the temperature where they turn from a liquid into a gas my mixture of liquids will come in at the bottom and it's going to be heated at the bottom and my fractionating column is going to develop a temperature gradient so all that means is it's very very hot at the bottom and it's much cooler at the top because it's further away from my heat source everything is going to evaporate at the bottom and then as things move up the column and it gets cooler and cooler they're going to turn back into liquids and they're going to turn back into liquids at different temperatures depending on what that boiling point is so in the example of crude oil natural gas is just going to come out the top of the fractionating column it's never going to turn back into a liquid because it doesn't get cool enough whereas things like petrol and diesel and naphtha are going to condense at different points down my column and they're going to be able to be separated like that our final separation technique is chromatography so you've probably done paper chromatography yourself and so you've done a little setup like this so you have a stationary which in this instance is a piece of chromatography paper and you have a mobile phase which is your solvent which is usually water if you're doing this in class so you're going to have a pencil line you always draw a pencil because pencil doesn't run and on that pencil line you're going to put the sample that you're interested in analyzing and you're going to put some things you want to compare it to so in this example from the exam board we've got five standards labeled ABCDE and you're going to put that chromatography paper into your solvent and the solvent will rise up the paper and it's going to take through different colors or different dyes with it and based on their solubility it's going to release them at different points and then you're going to do a comparison so you're going to look at your sample and look at these standards ABCDE and see well which one's match up have I got the same color that I have in the sample a then we move on to the structure of the atom so you need to be able to label the different parts of an atom you need to know that the protons are the positive particles that are in the nucleus in the center the neutrons are the neutral particles so the ones with a charge of zero also in the center and the electrons are these very very small negative particles traveling around the outside in the shells now there are a couple of other things you need to be aware of one is that you are supposed to know the general size of an atom so on average it's about North Point 1 and nanometers and you also need to know that despite how we usually draw them ie with a really big nucleus so you can see it the nucleus is actually only one ten thousandth of the diameter of the atom so that's not going to be an issue for you drawing it it's fine if you were going to draw an atom to draw the nucleus really big but just be aware of that for answering questions also one mistake I see students making a lot is that they will write that the relative mass of the electron is zero now that she's not true you don't need to know exactly how big it is it's about one over 1840 but you do need to know that it has a mass it does not have a mass of zero it has a mass of very very small you will also need to be able to look at a periodic table square and calculate the number of protons electrons and neutrons and if that's something you're still having trouble with I've got a whole video about that so you might want to go watch that now when we start looking at atoms a little bit more detail what we find out is that there isn't just one version of the cutter of a particular element so say if I look at the periodic table Square carbon it says that carbon has a mass of 12 and that it has an atomic proton number of 6 and that would tell me that that atom has six protons six electrons six neutrons now that is true for a vast vast majority of carbon but it is not true for all carbon the thing that defines carbon the thing that makes carbon carbon is that it has six protons not that it has six neutrons so there is a very small amount of carbon in the world that has seven neutrons and actually there's a very small amount of carbon in the world that has eight neutrons and we call those different versions of the atom isotopes so isotopes are atoms of the same element that have different numbers of neutrons but they do have the same number of protons you're all hopefully already aware that those isotopes exist and you've probably noticed before that chlorine on your periodic table has a mass of 35.5 and the reason for that is because actually chlorine has two stable isotopes the periodic table we just forget about the carbon 13 and the carbon 14 and we say it's got an ax of twelve for chlorine because there is quite a lot of chlorine 37 and quite a lot of chlorine 35 we have to take into account both of those isotopes so we report them both but you do need to be able to take some information from the exam board and word count what that accurate relative atomic mass of a bunch of different isotopes is this looks really scary but it's really really not so say for example the exam question says to me oh you've got a sample and it has 90 percent carbon 12 and 10 percent carbon 13 what is the mass of that carbon all you need to do is work out what's 90 percent of 12 what's ten percent of 13 hopefully you know that to work out 90% of 12 you can just do naught point 9 times 12 and to work out 10% of 13 you can do nor point 1 times 13 if you add those together you can work out that the relative atomic mass of that sample of carbon is 12 point one so the next part of the GCSE gives you an opportunity to look at how over time as we gain more we can change a scientific model and we use the atomic model to talk through this now this is content that can come up in your DCC chemistry and also your GCSE physics so it is well worth your time to really make sure that you know it so we start off with something that you might have heard called Dalton model but basically it's this idea that these atoms are indivisible spheres they're just like little hard billiard balls like snooker balls and this first picture it's an element because the snooker balls they're all the same color that are all the same size they are the same each other so that's what we called the Dalton model along comes JJ Thompson and he finds that actually these these billiard balls they're not indivisible they're not unseparated ball into little bits they have something called electrons they have these little bits of negative charge and he doesn't know where they are so he develops this thing called the plum pudding model and you might just about be able to see they've got little dots on them they look a little bit like a dice so he had this idea a plum pudding is kind of like a Christmas pudding it's just a big ball of positive charge and then studied in that are these electrons and that was his model then along comes Ernest Rutherford and his students and they did an experiment called alpha scattering experiment so you hopefully know from your GCSE physics that an alpha particle it's one of the types of radioactive decay two protons two neutrons and what they were doing was they were firing this at this very thin sheet of gold foil and what they were expecting is that these alpha particles would just kind of push their way through the sheet and it would be the same all over what they actually found was that a very very small number of these alpha particles were being bounced straight back the way they'd come they'd hit a small dense thing and what they realized was that actually mostly atom was empty space and there was this tiny bit in the middle called the nucleus so then we have a model well we've got this tiny dense positive nucleus and all these electrons around the site now you're probably looking at that third picture and going wait there's way too many electrons in that first shell well yes there is and that's what Niels Bohr came up with so he had this idea that electrons were gonna fly in shells they're gonna have distinct spaces that they could be in and that they would be at different is from the nucleus and so that's where we get our shells from a little bit after that people started to realize that actually it wasn't just one solid mass in the middle of the atom in the nucleus and that protons were things so then we start breaking up this central nucleus into protons and then finally 1932 along comes James Chadwick and he realizes that there are these neutral masses in the center of the nucleus along with protons so finally we have protons and neutrons in the nucleus and now we've got something that looks a bit more like it's a model of an atom that you've been taught today so next we move on to the periodic table you need to know a little bit about how it's arranged and also how it used to be arranged and how we've kind of moved on from that and also a few different groups within it so first up we need to know that the elements today are arranged in order of their atomic number so that's the number of protons that they've got and the reason it's got this slightly funny shape is to make sure that the elements that have the same properties are in columns together and the columns are known as groups so if I look at lithium sodium potassium rubidium cesium francium they have very similar chemical properties they react in the same way with oxygen they react in the same way with water the reason that the periodic table is called periodic table is because the same properties occur periodically that means at regular intervals so if you look at the first couple of rows we have eight different types of element and then when you get to the ninth element we go back to the first type now the reason that all of the elements in Group one have the same chemical properties they all behave in the same way and react in the same way is because they have the same number of electrons in their outer shell so if you're in Group one you have one electron in your outer shell if you agree to three you have three electrons in your outer shell and that is what is going to determine how you react with other elements periodic table hasn't always been laid out like this so when it was originally devised we didn't know about protons and neutrons and electrons so we couldn't say I will just put all of the elements that have one electron in their outer shell in Group one so initially the elements were arranged according to their atomic weight and that led to a few problems because although the weight does go up as the atomic number goes up there are a few little exceptions to this so for instance if you arrange things based on their atomic weight then you would put potassium and argon the other way around and you'd have potassium which is a really reactive metal in the middle of the noble gases and argon which is you know a noble gas it's super unreactive in the middle of this highly reactive group of metals and that's clearly not right also there were some things missing so there were some elements that when the periodic table was formed and the chemists at the time they didn't know that they existed now you've hopefully heard of Dmitri Mendeleev very famous Russian chemist and what he did was say I'm gonna leave some gaps because maybe there are things we don't know yet and he left these gaps but also he made predictions about the elements that would be discovered and thought those gaps and lo and behold he turned out to be correct so then we move on to splitting the periodic table up into different categories of elements and saying the characteristics of this category are that they they do these things and the characteristics of this category is that they do these things so the first two categories that we can split the periodic table into are the metals and the nonmetals you can draw this little staircase and the general rule is that anything on the left of it is a metal and anything on the right of it is a nonmetal and you really shouldn't struggle with remembering that because if you're panicking in an exam there are some metals that you know what they are right you're not going to forget that iron is a metal you're not going to forget that gold is a metal and you're not going to forget that oxygen is a non-metal so just look at where those elements you're really familiar with are and you'll remember really easily which side is which now metals are defined as elements that will react to form positive ions and nonmetals are the elements that will react and won't form positive ions you can see just looking at this that the vast majority of the elements are metals now you do need to be able to give just some examples of some properties of metals so describing words that we can use that are true for these metals so we've said they form positive ions we can also say that they are malleable that means that they can be hammered into shape because those ions can slide over each other they are conductive they conduct heat and electricity and that's because of their delocalized electrons and also generally speaking they have high melting points because of the strong metallic bonding in them so the only example of a metal that isn't a solid at room temperature he's mercury so next we move on to looking at three particular groups within the periodic table Group one group seven and group zero so Group one everybody's favorite the alkali metals these are the really really reactive ones that are very cool when you put them in water now you've almost certainly seen lithium and sodium and potassium being put in water and you hopefully remember that as you go down the group they get more exciting more impressive and you see them floating on the top of the water because they're really not very dense they're less dense than water that's why they float and you'll have seen bubbles of hydrogen gas being produced and you'll hopefully even have seen that when you do this with potassium it produces a lilac flame because the hydrogen ignites on its own and when potassium burns it burns with the line of flame you might also have seen that when you do this if you've got some universal indicator in the water the water goes blue and the reason for that is the reason that called the alkali metals when they react with water they produce a product a metal hydroxide and that metal hydroxide is an alkali so the metal itself is not an alkali but it reacts to make an alkaline you'll probably also seen if you saw this live that those three metals are usually kept under oil to stop them from reacting with oxygen so if you cut a piece of sodium or a piece of potassium in half inside it's really really shiny very quickly it goes almost black because it reacts with oxygen and makes this dark oxide the next group that you need to be able to talk about are group 7 which are called the halogens and because they're in group 7 they all have 7 electrons in their outer shell and they consist of molecules made of pairs of atoms so in the second part of the course you'll have drawn chlorine but if you need to draw any of them if you're drawing fluorine or iodine you do exactly the same thing you have 7 electrons in the outer shell so they share one and then have 6 running outside but we'll get onto that now I sometimes like to think of group 7 as being like the opposite of group 1 so Group one gets more reactive as you go down the group Group 7 gets more reactive as you go up the group because the halogens at the top of the group are more reactive than the bottom they can displace them so if I've got a compound say ah we've got sodium chloride and I add some fluorine fluorine is higher up the group than chlorine so it can displace it and kick it out and take its place in the compound finally we have Group zero which you may know as the noble gases so this is helium and neon and argon and xenon and all the rest of them the elements in group zero have got a full outer shell and that makes them really really stable because the point of chemical reactions is to try to get yourself a full outer shell and if you've already got one then you don't need to be involved in any chemical reactions you need to know that the boiling points of the noble gases increase as you go down the group so the way I like to think about this is the reason that your boiling point increases is because you have stronger forces between atoms that need to be overcome so this is kind of bad chemistry but I like to think of it a little bit like gravity so a bigger planet has got stronger gravity it's going to pull things towards it a bit harder so if you take argon that's your third noble gas going down the group it has bigger atoms than helium right at the top and that means that the forces between those argon atoms are stronger than the forces between the little tiny helium atoms and that means that it's going to take more energy for you to overcome it so you have to heat it up more to get it to a gas and that's why it has a higher boiling point the next unit in the first chemistry paper is all about chemical bonds metallic bonds ionic bonds and covalent bonds often a question isn't going to tell you this is a question about ionic bonding so it's really important that you know when each of these bond types will occur it's all to do with whether the atoms that are doing the bonding are metals or nonmetals whether they're from the left-hand side of the periodic table or the right-hand side so metallic bonds obviously exists in samples of metals a compound that contains at least one metal and one nonmetal will contain ionic bonds and then a substance made of only nonmetals whether that's a compound or an element will contain karela bonds first up let's look at this metallic bonding so if you look at a metal what you're going to see is regular rows of positive ions that can slide over each other and that's why metals and malleable why they can be hammered into a new shape because those ions can slide right over each other they're surrounded by a sea of delocalized electrons delocalized just means that they're not attached to one atom so they're free to go wherever they want and they can carry charge and they can carry energy and that's why metals are such good conductors of electricity and of heat now the electrons obviously negative and those positive ions they're oppositely charged so they're held together by a strong electrostatic force of attraction and that means that with the exception of mercury metals are solids at room temperature because they're held together by this really really strong force and that would take a lot of energy to overcome it what you have to remember is that the state something's in at room temperature is all down to whether it's a bubble below its boiling point and its melting point and to make it change state we have to give it more energy so if room temperature is enough energy to push something over its melting point then it'll be a liquid and if room temperature is enough energy to go past something's boiling point then it will be a gas and the amount of energy that a substance needs while that's down to the strength of the forces holding it together so because that metal has such strong forces between it it's going to be a solid now you also need to know about substances called alloys those are mixtures of metals or sometimes from one metal mixed with some carbon now the important thing about that mixture is that the positive ions in that mixture all the atoms of its carbon they're different sizes to each other so instead of having these lovely regular rows of positive ions the whole shape gets distorted and the layers can't slide over each other anymore and that makes alloys much harder that's why you have alloy hubcaps on a car because they're much much harder so they're much less likely to get damaged it's also why lots of jewelry is made out of alloys rather than pure metals if you have pure 24-karat gold it's actually quite soft it's very easily damaged whereas if you have none in karat gold the majority of that is actually made up with other metals and so it makes it harder and makes it more hard wearing our second type of bonding is ionic bonding and remember this is going to happen between a metal and a nonmetal and it's all to do with the transfer of electrons the giving and receiving of electrons now you need to remember that the electrons are going to move left to right across the periodic table just like you read left to right so they're going to go from a metal to a nonmetal now all bonding is to do with elements becoming more stable more energetically stable and elements are at their most stable when they have a full outer shell so for metals which usually only have one or two electrons in their outer shell energetically the best thing to do is to give those electrons away whereas for non metals which are kind of missing an electron or two the best thing for them to do is to take those electrons on now in the atom before this bonding starts there are equal numbers of protons and electrons positive and negative charges so as soon as you stop moving electrons around those charges aren't going to be balanced anymore so if we take this sodium here to start with in the Atem it has 11 protons and 11 electrons so when it loses one electron one negative electron it still has 11 positive charges in its nucleus but now it only has 10 negative charges to balance those out and so overall it has a single positive charge on the other hand this chlorine atom when it gains an electron it still only got 17 protons but now it's got 18 electrons so one of those electrons is not balanced out and that's why the chloride ion has a single negative charge you need to be careful when describing ionic bonding about how many atoms of each kind it's going to take to balance out so say instead of having a sodium atom I had a magnesium atom and it needed to lose two electrons it couldn't give both of them to this one chlorine atom because chlorine can only take one electron it only has one space in its outer shell so I would need two chlorine atoms to balance out that single magnesium atom now once this transfer of electrons has taken place and I have a positive iron and a negative iron those are going to join together to form a giant ionic lattice it's giant because this is never going to happen with just a couple of atoms right this is gonna happen with thousands and thousands of atoms at the same time and they're going to make this huge lattice the giant ionic lattice is held together by a strong electrostatic force of attraction and it's acting in all directions my lattice here is only drawn in two actions but remember this is going to be a 3d structure so that positive iron is going to feel a force going up and down and forwards and backwards and left and right it's a really really strong force so again ionic substances are solids at room temperature because room temperature doesn't provide them with enough energy to surpass their melting point because this giant ionic lattice is made up of charged particles these positive and negative ions it can conduct electricity but in order to conduct electricity that ions need to be free to move otherwise they can't carry the charge anywhere so when it's solid the ionic compound is not going to conduct electricity but as soon as we melt it by giving it energy or dissolve it in water so that is aqueous then it will be able to conduct electricity our third type of bonding is the covalent bonding and this will occur between pairs of nonmetal atoms when they share one or more pairs of electrons it's always pairs if you've drawn one electron on its own in a covalent bond and then something has gone terribly terribly wrong and you need to look again now there are two types of covalent substances there are small covalent molecules which you might seen referred to a simple covalent molecules if you've got an older revision guide and there are giant covalent structures which you might have seen referred to as macromolecules there are eight small covalent molecules that you need to be able to draw seven of these were in the old GCC but nitrogen is new so I would pay particularly close attention to that in case you have an old revision guide that doesn't include it in all of these small permanent substances what you're going to see is in the covalent bond there is a pair of electrons and we draw that with a dot and a cross to show that one electron has come from one atom and one electron has come from the other in all cases all of the atoms are going to have a full outer shell so the hydrogen that means it's going to have two electrons in total and then for every other type of atom it's going to have eight the ones to watch out for are oxygen which has a double bond and nitrogen which has a triple covalent bond which is what makes nitrogen super unreactive small component molecules like oxygen or nitrogen or methane tend to be gases or liquids at room temperature now the reason for that is that holding those molecules together isn't a bond isn't something really strong it's just something called a weak intermolecular force and it's really easy to overcome that weak intermolecular force because it doesn't take very much energy the bigger and molecule is the stronger the weak intermolecular forces between it will be that means that as small covalent molecules get bigger and bigger they do start to become solid so for instance if you think about a polymer that's got thousands and thousands of atoms so actually the weak intermolecular force can become quite strong but you need to be really really clear that it is not the covalent bonds break when these become liquids and gases these small covalent molecules can't conduct electricity because they haven't got any charged particles there aren't any delocalized electrons and there aren't any ions we just mentioned polymers polymers are very long chains of repeating units and we call those repeating units monomers those monomers are held together by strong covalent bonds between them to form a really long chain but between one really long chain and the next very long chain there's going to be a weak intermolecular force because the polymer chains are so long the weak intermolecular forces actually get quite big there's still nowhere near the size of an ionic bond or a covalent bond but they're enough to make many polymers solids at room temperature finally you need to know about giant covalent structures and there are a few different examples that you need to be familiar with so these are structures that are made up of usually thousands of atoms held together by strong covalent bonds it's not like a small component molecule where we've got maybe three or four atoms bonded together and then just a weak intermolecular force between them and the next three or four these are all bonded together so your first example is diamond and diamond is one version of carp in one form that carbon can take on wet bonds in diamonds every carbon atom makes four strong covalent bonds now it's really really common for diamond to be compared to graphite in an exam question and so often we see students write diamond has four strong covalent bonds no it doesn't diamond has thousands and thousands of strong covalent bonds but every carbon atom in it makes for strong covalent bonds and it's really really worth your while to use that word strong because often strong is an extra mark over and above just saying a curved bond because it has all those really really strong covalent bonds that atoms and diamond can't slide over each other like the positive ions in the metal and so that makes diamond incredibly hard like a small covalent molecules it doesn't have any delocalized electrons and it doesn't have any ions so diamond can't conduct electricity now in contrast to that graphite is still made of carbon but in graphite every carbon atom only makes three bonds and that means that it's got 1 electron leftover per carbon atom that isn't part of the bonding so that electron becomes delocalized it becomes free to move and carry the charge and that is why graphite can conduct electricity even though it's a nonmetal now because graphite is missing that extra set of bonds because every car bottom is only making three bonds not four bonds it forms these sheets these sheets made up of hexagons and those sheets aren't bonded together there's just a weak intermolecular force between them so they can slide over each other and that makes graphite really soft and slippery and so it's really useful as a lubricant particularly somewhere that you might not want to put oil because it's getting too hot and there's a risk that the oil might set on fire you also for the new GCC need to know something about graphene now graphene is just a single layer of graphite it can still conduct electricity and it's a really exciting material because we might be able to use it in all sorts of things like bendable electronics your third and final version of carbon is something called a fullerene now fullerenes are a type of nanoparticle those are really small particles with a few hundred atoms maybe one to 100 nanometers across and they tend to form tubes and balls and other kinds of small particles you particularly need to know about c60 that's buckminsterfullerene which is made out of 60 carbon atoms joined together the other type of giant covalent structure you need to know about which has nothing to do with carbon is silicon dioxide or you sometimes hear it called silica it has a structure like this where every silicon atom is joined to four oxygen atoms and every oxygen atom is joined to silica atoms it basically has exactly the same properties as diamond so you need no because it has all these strong covalent bonds it has a really high melting point so it's really useful for things like lining the inside of a furnace and because it has those strong covalent bonds using up all of the electrons there aren't any electrons left to be delocalized so it also can't conduct the third unit covered by the first chemistry paper of AQA TCC combined science is quantitative chemistry which is basically just using maths to figure out useful things like how much product of reaction should make to do just about anything in quantitative chemistry you need to be confident using the relative atomic mass of atom this is really clearly labeled on the periodic table but so often we see students panicking in an exam picking the wrong number and then of course none of their calculations work the relative atomic mass of an atom is the mass of that element compared to 1/12 of the mass of carbon-12 the first part of quantitative chemistry is all about the conservation of mass conservation means keeping things the same so the law of conservation of mass says that no atoms are lost all made during a chemical reaction that means in a simple equation you have to have the same number of each element on the left and the right take this equation for photosynthesis there are six carbons on the left in the six carbon dioxide molecules and six carbons on the right in the glucose molecule this also means in terms of mass the mass that atoms in a reaction can't change now sometimes it might look as if the mass does change and if this happens you can guarantee is because there's a gas involved say I have six grams of magnesium and I grind it up to a really fine powder and burn it at the end of that chemical reaction I should have 10 grams of magnesium oxide now that's not because I've magically created extra atoms it's the closet oxygen which used to be flying around in the atmosphere and not pressing down on the weighing scales is now stuck fast to that magnesium so we're weighing atoms existed before but they weren't being weighed likewise if I take five grams of calcium carbonate and I heat it up until it thermally decomposes I'll be left with two point eight grams of calcium oxide because some of the atoms in that calcium carbonate have been released into the atmosphere and they're no longer being weighed they still exist they're just not in my little pot in the weighing scales anymore the first real calculation that you're going to do is part of quantitative chemistry is relative formula mass shown by the symbol mr you have to be able to do this to do almost anything else in quantitative chemistry all you're doing is taking the relative atomic masses from the periodic table and adding them up so if sodium oxide there are two sodium's so 2 times 23 and 1 oxygen so 1 times 16 add it all up and you get 62 if you need a little bit more practice doing that I have got a whole video of practice calculations now for higher-tier only you need to be happy using something called a mole a mole is just a really big number you know that a dozen means 12 and a million means one with six zeros after it well a mole is six point naught 2 times 10 to 23 and we sometimes call that number Avogadro's number or Avogadro's constant it represents the number of atoms in the relative atomic mass of a substance in grams so say the number of atoms in 12 grams of carbon or 16 grams of oxygen or one gram of hydrogen that's also the same as the number of molecules in 44 grams of carbon dioxide or 18 grams of water once you know what the relative formula mass of a substance is you can start saying well how much does five moles of it weigh or you can start saying well if I've got 220 moles of carbon dioxide then how many moles is that again if you're not super confident about that I have got another video with a few practice calculations imagine you're buying apples and the price is three pounds per dozen you have 18 pounds how many can you buy well you do 18 pounds divided by 3 pounds and you'd get six doesn't exactly the same idea here you can also work in the other direction say I want to know how much four dozen apples would cost me four times three pounds is 12 now once you're happy with both mr.and moles you can start doing something useful say I've got 7 tonnes of copper ore and I want to know how much copper I can extract I need to use the relative masses of the substances involved and the balance symbol equation to work out something called predicted yield or the expected yield so let's take one example I've got 510 grams of aluminium oxide and I want to know how much aluminium I can make from that first thing I need to do is work out how many moles of aluminium oxide I've got and in order to do that I need the M R the relative formula mass so just like we said we're just gonna add up the masses of all the atoms in the compound we've got two aluminium atoms each with a mass of 27 we've got three oxygen atoms each of the mass of 16 so I add together 2 times 27 plus 3 times 16 and I get 102 so now I know that one mole of aluminium oxide would weigh a hundred and two grams but I don't have a hundred and two grams I have 510 so I do 500 and 10 divided by 102 and that tells me I've actually started with five moles now I need to go back to my symbol equation what my simple equation actually tells me is the ratio of the different chemicals that are taking part in this reaction all being formed so you can see that aluminium oxide doesn't have a coefficient it doesn't have a big number in front of it but aluminium has a two so what that tells me is how if many moles of aluminium oxide I started with I'm gonna make twice as many moles of aluminium so if I started with five moles of aluminium oxide I must be going to make 10 moles aluminium now I just need to know what the relative mass of aluminium is so I go back to my periodic table I see that it's 27 and then 10 moles times 27 grams per mole weighs 270 grams the next calculation for quantitative chemistry is concentration if we leave the numbers out for a second concentration is just a measure of how many dissolved particles are in a set volume of solution so we call those dissolved particles the solute and we call the liquid that they're dissolved in the solvent now our units for concentration are grams per decimeter cubed a decimeter cubed is exactly the same thing as a liter and it contains a thousand centimeters cubed and quite often you might get a sneaky question but they give you the volume and centimeters cubed and expect you not to notice so make sure you keep an eye out for that one now if you know that the units are grams per decimeter cubed without slash though in the middle well hopefully you know from math so that slash is really a divide sign so what we've got is grams divided by decimeter cubed or grams as a measure of mass and decimeter cubed as a measure of volume so we're doing mass divided by volume you've almost certainly encountered the idea of limiting factors in biology where you study photosynthesis that idea that the limiting factor whatever is slowing photosynthesis down so it could be that there's not enough carbon dioxide or it could be that there's not enough light or it could be that the temperature is too high or too low in chemistry we have a very similar idea limiting reactants so this is the idea that if I've got two chemicals reacting together one of them is going to be in excess there's going to be too much of it and one of them will be the limiting reactant the one that there's not enough of this reactant is going to be completely used up in the reaction so imagine I've got a really big beaker full of acid and I put a tiny piece of magnesium in it the magnesium is going to react completely and disappear and that means that that one is the limiting reactant the fourth unit you need to know about is chemical changes this starts the reactivity looking at how metals react with oxygen with water with acid and then using those reactions to place them in an order of reactivity you need to know that metals react with oxygen to form compounds called metal oxides so this could be a simple question where you have to complete a word equation like saying 10 plus oxygen what does it make Oh tin oxide metals react with water to form metal hydroxides and hydrogen so similar thing lithium plus water goes to lithium hydroxide and hydrogen you should know that those metal hydroxides are alkalis so they'll turn universal indicator blue and you should know your gas test for hydrogen which is that if you ignite it it burns with a squeaky pop sound now very few metals are reactive enough to react with water in a way that you can see clearly so really you're thinking about your alkali metals maybe calcium magnesium if it's hot enough water so if you were trying to compare the reactivity of some metals if I put potassium in water there's a lot of bubbles that I can see whereas if I put copper in water I won't see any reaction at all so I could look at those and I could say potassium is more reactive because it produces more bubbles next you need to know that metals react with acids to form a salt which is an ionic compound and hydrogen so again you can test for that hydrogen using your squeaky pop test and you can judge the reactivity of those metals on how quickly it reacts with the acid how many bubbles you see you do need to be able to name those salts so there are three acids that you should know about for GC C hydrochloric acid sulfuric acid and nitric acid the salts that are formed are each gonna have two names kind of like a first name and a surname so if you put a metal in hydrochloric acid the name of the salt will be the name of the metal followed by chloride so for instance iron and hydrochloric acid will make iron chloride or tin and hydrochloric acid would make tin chloride sulfuric acid makes sulfates so magnesium sulfate zinc sulfate nitric acid makes nitrates you need to know that metal oxides can react with those same acids to form salts and water because the oxygen from the metal oxide is joining up with the hydrogen that you had when it you're just putting some metal in to make water metal carbonates react with those same acids to form salts water and carbon dioxide and you should be able to write word equations and symbol equations for all of those reactions remember to test for carbon dioxide you need to bubble it through lime water and the lime water will go milky or cloudy it turns white you can use all of these reactions to help place metals in order of reactivity your specification just mentions potassium sodium lithium calcium magnesium zinc iron and copper which is slightly fewer metals and we used to talk about in the old specification remember to talk about what observations you're making so what can you actually see it's not enough to say I think that magnesium is more reactive than iron because I saw it react faster you didn't see it react what you saw or maybe the bubbles of gas being given off make sure that in that reactivity series you know where carbon and hydrogen go so for instance you know that the only thing on that list that's less reactive than hydrogen is copper so if you put copper in acid you're not likely to see a reaction because it's not going to be able to displace the hydrogen metals are defined as elements that make positive ions so the reactivity of a metal is related to how easily it makes those ions if you have an element from Group one that only needs to lose a single electron to become a positive iron that's very very easy to do and that's what makes the Group one metals so react also the further you go down a group that easier it is for it to lose that outer shell electron and therefore the further down the group you go the more reactive the metals get now we can use this information about metal reactivity to think about how metals can be extracted so very unreactive metals like gold silver platinum copper things that you would make jewelry out of they are just found as elements in the Earth's crust we sometimes say they're found native slightly more reactive metals like iron and tin and lead can be extracted by reduction with carbon that basically means if you've got some iron oxide and you heat it up with carbon because the carb is more reactive it displaces the iron it steals away that oxygen impurity and so what you're left with is carbon dioxide and pure iron if you're sitting the higher tier then you need to be able to say in terms of electrons whether a reaction is an oxidation reaction or a reduction reaction the way to remember this is oil-rig which stands for oxidation is loss of electrons reduction is gain of electrons so if we take the reaction of magnesium with hydrochloric acid to make magnesium chloride and hydrogen if we just look at the magnesium on the left-hand side it's a magnesium atom it has the same number of electrons as protons 12 of each on the right-hand side when it's made a compound it's formed magnesium ions 2 plus ions and that tells us it has 2 more protons than electrons on the left it has 12 electrons on the right it only has 10 it's lost two electrons so we say that it has been oxidized the hydrogen on the other hand started off as hydrogen ions which have no electrons but one proton that's why they have a single positive charge on the right-hand side they've now regained that electron reduction is gain of electrons so the hydrogen has been reduced you also need to know about neutralization reactions reactions where the pH of a solution moves closer to 7 these reactions are going to include an acid a substance that produces hydrogen ions the neutralization could be by an alkaline which contains hydroxide ions alkalis are an example of a base so bases are any chemical that can neutralize an acid and some of them are soluble and we call them alkalis but some of them like metal oxides are not soluble they're insoluble we can determine how acidic or alkaline something is by using something called the pH scale a scale that goes from 0 to 14 you need to know a general equation for the neutralization of an acid by an alkali this is an equation that works for any acid and any alkaline so we don't include the spectator ions so we have h+ for the acid o h minus for the alkali joining together to form water if you're sitting higher tier you need to know the difference between strong acids and weak acids this is different to concentrated and dilute so strong acids are hydrochloric acid sulfuric acid and nitric acid their acids are completely ionized in water that means as soon as they dissolve they give up their hydrogen iron weak acids like citric acid ethanoic acid and carbonic acid are only partially ionized so some of those particles when they dissolve will give up a hydrogen iron but some of them will remain intact finally for unit 4 you need to know about electrolysis this is a pretty big subject and if you're not sure about it I have got a whole video about electrolysis electrolysis means splitting ionic compounds using electricity it has to be an ionic compound because a covalent compound doesn't have overall charged particles that will be able to move in response to the electricity the ionic compound is called the electrolyte and it needs to either be melted or dissolved in order for the ions to be able to move we call those ions cations and anions the cation is the positive one and the anion is the negative one those ions are going to move in response to charge the positive cations move to the negative cathode and the negative anions move to the positive anode this is because opposites attract when they arrive there they will be discharged which means they'll stop being ions and turn back into atoms this happens by they're either gaining or losing electrons if you're sitting hired here you need to be able to write 1/2 equations for these discharge processes so for instance the cation aluminium has a three plus charge therefore it needs to gain three electrons in order to turn back into an aluminium atom this is called reduction oxide ions are an example of an anion when they reach the anode they need to lose their once they're oxidized be really careful with these anions firstly they have names that are slightly different from the names of the element so for instance you have an oxide iron even though the element is called oxygen also when these anions are discharged and turned back into elements they usually go round as pairs so for instance oxygen goes round as Oh - that means when you write the half equation you need to remember to write two oxide ions and you also need to remember that four electrons will be lost in total the extraction of aluminium is named as an example of electrolysis so you need to know that it's extracted from aluminium oxide which is mixed up with cryolite to help to disrupt the lattice you should be able to talk about how this is a very expensive process because it takes a lot of energy to melt the aluminium oxide it takes a lot of energy to provide the electricity to do the electrolysis and also because the electrodes which are made of graphite will gradually wear away and need replacing over time you also need to know about the electrolysis of solutions these solutions will contain the ions from the compound but also hydrogen ions and hydroxide ions and the hardest thing to remember is which ions will be discharged there are three rules you need to know firstly the most reactive cation will stay in the solution that means that the only metals you can extract by electrolyzing a solution are your jewelry metals copper silver gold platinum if there's any other metal in the solution then it's more reactive than hydrogen and instead of the metal being extracted it will be the hydrogen that's released your second rule is that halides are preferentially discharged so those are ions that have come from group 7 if there are no halide ions in your solution then the hydroxide ion will be discharged and that will release oxygen the fifth and final unit in the first chemistry paper of aqueous GCC combined science is energy transfers you need to know that energy is conserved in a chemical reaction which means that overall it stays the same so over the course of that reaction the energy can move around and be stored in different places so it could move from the reactions to the surroundings and warm them up or it could be taken in from the surroundings and you could end up with products that had more energy in them and the reactants you started with but overall it has to remain the same we can use temperature changes to tell us whether a reaction is exothermic or endothermic you almost certainly know that you go out of an exit and that therm usually refers to heat so I think like a thermometer so you can think of an exothermic reaction as one that probably gives out heat now it could be other types of energy it could be light it could be sound but in the majority of cases we are talking about heat and if you get given a quantitative question where it's got some numbers it's almost certainly going to there are three named examples of exothermic reactions that you need to know combustion so burning things oxidation reactions that could be say burning a metal in oxygen and neutralization so when we mix it cuz it and an alkali together in terms of what they're used for you need to know that they can be used for self heating cans the kind of cans that you might take camping with you or for hand warmers you can think of endothermic reactions as being like the opposite of an exothermic reaction because they take in energy from the surroundings so generally speaking they're going to make the surroundings cooler your specification mentions thermal decomposition so that could be something like heating copper carbonate with a Bunsen burner so that it breaks down into copper oxide and carbon dioxide reaction of citric acid and sodium hydrogen carbonate and also it's not explicitly measured in your spec but it's probably worth knowing that photosynthesis is another example of an endothermic reaction because of course it takes in energy in the form of sunlight endothermic reactions can be used for those cooling packs that you might use instead of an ice pack there is a required practical in this unit and it involves investigating the variables that affect the temperature changes in reacting solutions so there isn't one specific named example here there are lots and lots of different experiments that you might have done in your class and the exam board can just give you some information about a new situation you haven't met before so they could ask you to write a method they could ask you to interpret some data so if you're asked to write a method there isn't a specific technique that you need to know about but you need to talk about things like the fact that you're probably going to do this reaction in a polystyrene cup or some kind of insulated vessel to try and stop heat changes you're probably going to want to use a digital thermometer because that's likely to have a higher degree of precision than a thermometer that you just read yourself because it can probably go to a couple of decimal places whereas if you're just reading it by eye you're not really going to be able to be more accurate than a half a degree C and you're going to want to mention control variables so things like if you were going to measure the temperature change when you add some metal to some acid you would want to keep the volume of the acid the same and the concentration of the acid the same and the size of the piece of metal the same they could give you some data from an experiment like this and ask you to just say whether it's exothermic or endothermic so if the temperature on the thermometer goes up it's exothermic if the temperature on the thermometer goes down it's endothermic you need to be able to discuss reaction profiles which are these diagrams which show you how much energy the reactants and the products have now you're know from unit six that in order for reactants to react they have to collide with each other which means bang into each other and they need to have a certain amount of energy called activation energy so what that means is that there any reaction to start the particles need a certain amount of energy in order to get up to this transition state here they need to acquire some energy from somewhere and it might be that they can take in that amount of energy from their surroundings in which case the reaction is going to happen pretty much spontaneously or it might be that that's quite a lot of energy and you need to heat the reaction to start it off whatever that amount of energy is it's called the activation energy so if you're labeling one of these reaction profiles you want to label the activation energy from the height of the reactants up to the height of the transition state now the other thing that you can do these reaction profiles is tell whether a reaction is exothermic or endothermic and this is really straightforward once you know what you're looking for but it can just be slightly counterintuitive the first time you meet it so the line on the Left represents how much energy is stored in the reactants how much chemical energy is stored in their bonds and the line on the right is how much energy is stored in the products now you can see in my first example here that the reactants have more energy than the products well we said at the start of this section that energy is always conserved there must be the same total amount of energy so if you imagine my reactants have 200 kilojoules of energy and my products only have 100 kilojoules of energy well where did the spare hundred go it was given out it went out into the surroundings and so my surroundings would have got warmer and that makes this an exothermic reaction so in every exothermic reaction you're going to see your reactants being higher than your products if we look at my second example this is the exact opposite so this is an endothermic reaction we start off with our reactants only having say 100 kilojoules of energy and then our products at the end have got 200 where if they got that extra hundred from well they've taken it in from the environment around them they've taken it in from the surroundings so the surroundings will have got cooler because now there's more energy stored in the chemicals now if you're taking higher tare you also need to be able to do bond enthalpy calculations and work out what the energy change of a reaction is now my number one piece of advice for this would be that when you're doing one of these questions if they haven't drawn out the reactants for you draw them yourself because the number one mistake that we see is that the students do the calculation completely right but they miss count the number of bonds that are being made or being broken and so they get the calculation slightly wrong and they come out with completely the wrong answer now the first thing that you need to know is that breaking bonds needs energy it takes energy in and making bonds will release energy be really really careful that you're not accidentally saying that the energy is made because it's not energy was already there it was already in the atoms but when they bond it's going to be released if you're struggling to remember this I like to think of energy as being a bit like money so imagine you go to a shop and if you knock something over and break it you have to pay for it you have to give them some money and they take that money away because you've broken something so if you break a bond it takes in the energy but then if I make something and I take it to the shop for them to sell they will give me some money for it so when I make something I get the energy back so the exam question is going to give you a symbol equation like this and what I've done is I've drawn out my molecules now the helpful thing about having drawn that is that I can now see really really clearly that I have two hydrogen hydrogen bonds one oxygen oxygen bond and four oxygen hydrogen bonds those bonds all have different bond energies and they will be given to you in the question now looking at my equation in order for this reaction to happen the hydrogen hydrogen bonds need to break and the oxygen oxygen bond needs to break so when those bonds break some energy will be taken in I can use the bond energies that have been given to me in the question I can see that each one of those two hydrogen hydrogen bonds will require 436 kilojoules per mole to break and then the oxygen will require 498 kilojoules per mole to break so if I add all of those up I can find out that this is going to take me 1370 kilojoules to do my next step is going to be to make some bonds so again I look at the bond energies they give me in the question and I can see that an oxygen hydrogen bond has a value of 464 so you can write down four times 464 or you can just write down the value of each bond as you color it green and if I add those up I find out that the energy being released is going to be 1856 kilojoules now the final part of this question is going to be for me to calculate something called the Delta H and that's the overall energy chain so what you want to do is to work left to right across your page just like you are reading the equation and you're going to do the energy taken in take away the energy that's released so in this example it's one thousand three hundred and seventy take away one thousand eight hundred and fifty six which comes up with minus 486 now we saw just a little bit earlier with those reaction profile diagrams that whenever you have an exothermic reaction energy is lost to the surroundings there is a negative energy change so this negative number here tells me that burning hydrogen in oxygen is an exothermic reaction