this is a revision video for aqa gcc chemistry paper 1. it covers all of the content for this paper although not in a huge amount of detail as otherwise the video 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 to do a last minute cram before the exam if you're taking gcc combined science you can watch out for the green headings whenever there's triple science content so that you know to skip or make use of the time stamps in the description below the first topic in aqa gcc chemistry is all about elements and compounds and in order to understand about these you need to know a little bit about atoms too all substances are made of atoms and an atom is the smallest part of an element that can exist there are about a hundred of these you don't need to know the exact number for gcse chemistry and these are shown in the periodic table when we start talking about compounds these are substances are formed by chemical reactions where different elements join together a compound contains two or more different elements which have been chemically combined and that's just your exam board's way of saying bonded and they've been chemically combined in fixed proportions that means that they're always in the same ratio to each other and therefore we can represent a compound using a chemical symbol formula like h2o you do need to be able to name some of these compounds and there are two rules that we're going to use to do this the first one applies to ionic compounds that contain one metallic element and one non-metallic element so the name of the metal just goes in exactly as it is and then you take the first syllable from the non-metal so the first sound in that word and you put ide on the end so examples of this might be iron oxide copper chloride or tin sulfide then if we have a compound which contains three elements and the third one is oxygen we go through the same process but instead of putting ide on the end we put eight and that eight is like a chemist's secret handshake that tells you that the compound also contains oxygen so examples of this might be copper sulfate silver nitrate or aluminium phosphate in each instance we have the name of the metal just as it is the first syllable of the first non-metal and then the eight to show us that there's oxygen in there too some substances aren't pure elements or pure compounds they're mixtures so when we say a chemical mixture we're talking about two or more elements or compounds that are not chemically combined together and that means that they can be separated by physical processes rather than chemical reactions you need to be able to discuss five different physical processes that can be used to separate mixtures the first of these is filtration which is used to separate an insoluble solid from a liquid in filtration we're going to use a piece of filter paper usually inside a funnel and then when you pour your mixture into that the insoluble solid because it can't dissolve it can't go through the filter paper so it's left on the top as a residue whereas the liquid is going to go through the filter paper and we call that the filtrate if you have a soluble substance like salt or copper sulfate then filtration is no good because that soluble substance would be able to go through the filter paper along with the liquid that it's dissolved in so instead we use crystallization in this you're going to put your solution into an evaporating basin and then you're going to gently heat it so that the water or other liquid can evaporate and what's left behind is crystals of the soluble salt that you had in the liquid in the first place distillation can be used to separate mixtures of liquids according to their boiling point so you might have seen certain spirits on them that say that they're double distilled and this is to do with heating up the mixture of liquids so that some of the water can be removed to make the alcohol stronger now we have simple distillation and fractional distillation in simple distillation you'd only be trying to separate out two different liquids whereas fractional distillation we're trying to separate out multiple different liquids but in each instance we're basically going to heat up the mixture until one of the liquids evaporates when it reaches its boiling point fractional distillation takes place in a fractionating column which is a very very long tube with a heat source something like a furnace at one end of it so the mixture of liquids is going to come in and everything is going to be heated up to a very high temperature and pretty much everything is going to evaporate as those evaporated gases move up the fractionating column there's a temperature gradient which basically just means the further away you are from the furnace the cooler it gets as that mixture of gases cools down the different gases are going to turn back into liquids or condense at different points because they have different boiling points and so as each one condenses and turns back into a liquid it can be removed we talk about this in more detail in unit 7 which is in paper 2. chromatography is a separation technique often used when we want to analyze mixtures of liquids such as colours in an ink to see what's there substances are separated according to their retention this means how well are they retained or hung onto by a stationary phase while a mobile phase passes through it so in this example of paper chromatography the chromatography paper is the stationary phase and the water or another solvent is the mobile phase in paper chromatography retention links directly to how soluble the substance is in the solvent that's being used to analyze the results your sample needs to be compared to a number of known standards these are chemicals which we've already identified and we already know the results for and by comparison we can see whether or not they're in your sample so if i was looking at these results i would say that this sample is a mixture of two substances standard a and standard b in paper chromatography it's important to remember that the start line must be drawn in pencil and also that the solvent shouldn't come above that pencil line both of these will stop the start line from running and obscuring the results you may also want to use a lid to stop your solvent from evaporating next we come on to the nuclear model of the atom you should be able to draw this model of the atom and identify that the protons and the neutrons are found in the nucleus at the center while the electrons orbit the outside you should know that when you're drawing a specific atom we can put two electrons into the first shell before moving on to the second and then we can put eight electrons into that second shell before moving on to the third and we can put eight electrons into the third shell before moving on to the fourth you won't be asked to draw any atom that's further along in the periodic table than calcium thinking more about these subatomic particles these smaller parts of the atom you should be able to identify how many there are in a particular atom by using a periodic table square here we have lithium the number at the bottom is the atomic number and it does actually say on your aqa periodic table in the key atomic brackets proton number so that tells you that for this atom there need to be three protons because the atomic number is three in an atom the number of positive protons is balanced out by negative electrons so if there are three protons there will also be three electrons the number at the top which is the mass number tells you the total number of particles in the nucleus so since we know that there are three protons and we know that there are seven in total and that the nucleus only contains protons and neutrons by taking three away from seven i can work out that there are four neutrons you also need to know the relative masses and relative charges of each of these particles for the relative masses protons and neutrons are both one which means they basically weigh the same as each other and for electrons we're going to say that it's very small if you have learnt a number that's fine but the important thing is that the mass is not zero it's very small or negligible but it is not zero for the relative charges the most important thing to remember is that as soon as you see that word relative you won't get the mark if you just write positive or negative we need to have an actual numerical value so protons with a p are positive and they have a charge of plus one the electrons which balance them out are negative and they have a charge of minus one and then the neutrons which are neutral have a relative charge of zero you should also know how small these atoms are they're absolutely tiny 0.1 nanometers across which we can also express a standard form so a radius of 1 times 10 to the minus 10 meters and even though in this diagram we've drawn the nucleus quite big that's really only so that i can fit the positive signs on actually that nucleus is 1 10 000 of the entire radius of the atom you should be able to describe that isotopes are atoms of the same element which have the same number of protons and different numbers of neutrons so these two lithium atoms here are isotopes because they both contain three protons but the top one has four neutrons whereas the bottom one has six neutrons you may be asked to work out the relative atomic mass of a sample containing two isotopes and this is actually much easier than it looks so let's say we're asked to calculate the relative atomic mass of a sample that contains 90 lithium-7 and 10 lithium-9 all you need to do is work out what 90 percent of seven is and ten percent with nine is and add them together and you can use whichever method you're more comfortable with personally i prefer decimals so i've done 0.9 times 7 which is 6.3 and 0.1 times 9 which is 0.9 and if i add those together i get 7.2 you should always do a little bit of a common sense check at this point this is a kind of weighted average so my answer is going to be somewhere between the two numbers i started with so between 7 and 9 and it's going to be closer to the isotope that i have more of so 7.2 is a reasonable answer because it is between 7 and 9 and because it's closer to 7. at this point we reach a bit of a history lesson so you should know that scientists all the time are doing experiments and accumulating more data and as a result changing their models and so at this point in the gcse we look at the different models of the atom over time and how scientists have updated them and changed them as they've learned more about atoms so we start with john dalton's model which is the model that you would have used when you first started drawing particle diagrams when you were instead of year seven or year eight so john dalton envisaged atoms as being solid spheres which couldn't be broken down any further and he compared them to billiard balls then a little bit later jj thompson was responsible for the discovery of electrons but he didn't know where in the atom they were and he envisaged them as being studied into a large ball of positive charge and he compared this to a plum pudding which if you've never seen a plum pudding just think of a christmas pudding with all the little sultanas in there and then imagine that instead of sultanas they're electrons so in the plum pudding we don't have a nucleus and we don't have electrons going around the outside we just have them stuck in the middle then came ernst rutherford and this is the one example of a scientist where you do need to know about the experiments he did in order to find out what he did so ernest rutherford and his students geiger and marston carried out the alpha scattering experiment in which they took alpha particles which are positively charged helium nuclei and they fired them through a very very thin sheet of gold foil what they found was that the vast majority of these went straight through and this was evidence that most of the atom was actually empty space but they also found that a small proportion were deflected so they were either bounced off to the side or bounced straight back and this was evidence for two things one was that there was a mass concentrated in the center and the second thing was that that concentrated nucleus was positively charged because that explained why the positive alpha particle was repelled once rutherford had identified that the atom had a central nucleus the next step was that niels bohr identified that the electrons weren't just moving randomly they were actually orbiting at fixed distances away from the nucleus and we now call these shells this led to some really confusing data particularly surrounding isotopes so in 1932 when james chadwick discovered the neutron this allowed us to explain why some elements appeared to have relative atomic masses that weren't integer numbers it was because there were two different isotopes and they were both being averaged out next we come onto the periodic table and the first thing you really should be aware of is that your aqa gcc periodic table does contain a key and it tells you that the number at the top is the relative atomic mass and the number at the bottom is the atomic number also known as the proton number so if you need to work out how many protons an element has use the key check that it says that it's the bottom number and that will be your number of protons now we can start to break this periodic table up into groups which are the columns and into periods which are the rows all of the elements in a particular group have the same number of electrons in their outer shell and this means that they're going to have chemically similar properties so everything in group one has one electron in its outer shell and therefore is a highly reactive metal everything in group two has got two electrons in its outer shell and so on the periods are arranged by elements that have the same number of shells so everything in the first period has one shell and everything in the second period has two shells we can also split the periodic table up into metals and non-metals by drawing a staircase either here or here but since for gcc chemistry we're only interested in the first four periods it doesn't actually make any difference for the groups that we're looking at which are groups one two six seven and also the transition metals the one you do need to be aware of though is that hydrogen despite being a colorless gas and quite clearly a non-metal is slightly towards the left hand side of the periodic table now you should know that the metals are found towards the left and the bottom whereas the non-metals are found towards the right and the top for gcse chemistry metallic elements are those that form positive ions and metals are malleable conductive and have high melting points which we'll discuss in more detail in unit two the reason that the periodic table is called the periodic table is because the same properties occur at regular intervals so if we count from lithium on eight elements we get to sodium which is very similar to lithium they're both alkali metals and then if we count on another eight elements we get potassium which is also an alkali metal and also very similar so because these properties come back around at regular intervals or periodically this is called the periodic table the periodic table hasn't always been laid out in the way that we use now when scientists were first starting to develop it subatomic particles like protons and electrons and neutrons hadn't yet been discovered so we couldn't put all the elements with one electron in their outer shell into group one and put them together because we didn't know how many electrons anything had we didn't even know that electrons existed and we couldn't put elements in order of atomic number because that's how many protons they have and proton wasn't even discovered until 1920. so initially elements were grouped together based on their chemical properties and then they were arranged according to their atomic weight and generally this worked pretty well because atomic weight tends to go up as atomic number goes up but it did lead to a few problems because there are some exceptions so for instance if you put the elements in order of their atomic weight then potassium and argon would be the other way around and so you'd have potassium which is this super reactive alkali metal in the middle of the noble gases and then argon which is this inert unreactive colorless gas in with a group of highly reactive metals like lithium and sodium and that's clearly not right also there were elements that were missing because we hadn't discovered every element at the point at which the initial drafts of the periodic table were being produced you've hopefully heard of the famous russian chemist dmitry mendeleev and he did a couple of things one thing was that where something just didn't make sense he did actually switch some pairs of elements over to make sure that they were put with more appropriate elements the other thing he did was that he said i'm going to leave some gaps because maybe there are elements we haven't discovered yet but he also made predictions about the elements that would eventually be discovered and fill those gaps so lo and behold when scientists did discover those elements and they also found out that they had the masses and the chemical properties that he had predicted this was taken as good evidence that his work was accurate and so it was widely accepted by the scientific community you also need to know about three specific groups in the periodic table and then if you're taking gcc chemistry or triple science you also need to know about the transition metals the group one elements are often referred to as the alkali metals and these are soft highly reactive metals you can see here that they are so soft that i was able to cut this one with my scalpel they all have one electron in their outer shell and when they undergo chemical reactions they lose this electron and because it's easier to lose an electron the further away it is from the nucleus because it's feeling less electrostatic attraction to the nucleus the larger the atom gets the more reactive it is so as we go down the group and add an extra shell for each period the atoms get larger and their elements get more reactive so for instance potassium is much more reactive than sodium which is more reactive than lithium when these metals react they can react with oxygen to form metal oxides and this is why before i cut the metal open it wasn't shiny on the outside because it was coated with a layer of metal oxide they react with water to form metal hydroxides which are alkalized so you can test for them with universal indicator and also hydrogen gas which you can ignite or set bio to and prove that it's hydrogen because it will make a squeaky pop sound and also the alkali metals react with chlorine which is a green gas and when they do that they form white solids which are metal chlorides and it's a very exothermic reaction so we also see a very bright light as the green chlorine gas disappears the next group of elements that you need to know about are group seven which are also known as the halogens so this is fluorine and chlorine and bromine and iodine and astatine and because they're in group seven they all have seven electrons in their outer shell so the next group of elements that we need to know about are group seven which are the halogens so fluorine and chlorine and bromine and iodine and astatine and because these are in group seven they all have seven electrons in their outer shell they all form divalent molecules so these are molecules that are made of two atoms covalently bonded together and because they all have the same number of electrons in their outer shell they all form these bonds in the same way in unit two the structure and bonding topic you will have practiced drawing chlorine but you can use the exact same principles to draw any of the other group seven elements as well so in each instance we're going to have each of the two atoms having six electrons of its own and then a shared pair of electrons forming that covalent bond in the center now in terms of their chemical reactions i sometimes like to think of the halogens as being the opposites of alkali metals so whereas the alkali metals lose an electron in order to react and therefore get more reactive as you go down the group and the atoms get larger the halogens gain an electron in order to undergo chemical reactions and therefore they get more reactive as you go up the group and the elements get smaller so fluorine is more reactive than chlorine and so on and so forth and what that means is that fluorine is able to displace chlorine so if you had a compound that contained chlorine say potassium chloride and you introduced fluorine to it the fluorine could take the place of chlorine in that compound and kick it out now also as we've just said as we go down the group these atoms get larger and therefore the molecules that they're being made into get larger and therefore the weak intermolecular forces between them gets stronger so this is also going to mean that the um whereas chlorine and fluorine are gases bromine is a liquid and iodine and astatine are solids because as we go down the group the melting points and the boiling points get higher if you're taking combined science and the last group of elements that you need to know about are group zero or the noble gases and that word noble means inert or unreactive which tells you what the key characteristic of this group is helium neon argon and so on are very unreactive and the reason for this is because their atoms have got a full outer shell when atoms undergo chemical reactions they exchange electrons in order to find a stable configuration and a full outer shell is one of the most stable configurations there is so because these elements already have a full outer shell they don't need to undergo chemical reactions and so the noble gases don't usually form compounds this makes them very useful for situations where we're trying to stop chemical reactions from happening so for instance if you have a light bulb and that metal filament is getting very very hot if that light bulb was full of normal air then the metal would react with the oxygen in the air and therefore the filament would break but if that light bulb is filled with a noble gas instead then that noble gas won't react with the metal filament and therefore the light bulb will last a lot longer you do also need to know something about the physical properties of these noble gases so as we've just said with group seven as you go down the group and the atoms become larger there's a stronger interaction between them there's a stronger force and that's harder to overcome and therefore the boiling point of the noble gases becomes higher as we go down the group if you're taking combined science you can just ignore this slide but if you're taking gcse chemistry or what you might call triple science or separate science then you also need to know about the transition metals this is the block of metals that sit between group 2 and group 3 of the periodic table and it includes metals such as chromium manganese iron cobalt nickel and copper in comparison with group one they're much harder and denser they have higher melting points and they're far less reactive with oxygen and water and halogens like chlorine they characteristically form ions with different charges so for instance iron forms iron two plus and iron three plus they form coloured compounds and the different colours correspond to the different charged ions and also they're very useful as catalysts the next topic in paper one of aqa gcc chemistry is all about bonds metallic bonds ionic bonds and covalent bonds now often a question isn't going to say to you this is a question about ionic bonding they're just going to name a substance and expect you to identify the type of bonding so it's really important that you can use your periodic table in order to work out what the type of bonding present is we've already talked about how we can split the periodic table into metals on the left and non-metals on the right as long as we remember that hydrogen up there in the middle is actually a non-metal now this is important because it allows us to identify what the type of bonding in a particular substance will be if all of the atoms in that substance come from the metal side of the periodic table then there will be metallic bonds if they all come from the non-metal side of the table then it's going to be covalent bonds and if we have some atoms from either side of the staircase then we're going to have a substance that contains ionic bonds the structure of a metal can be described as a giant metallic lattice this is made up of regular rows of positive ions satin layers that can slide over each other surrounded by a sea of delocalized electrons electrons are described as delocalized when they aren't tied to one particular atom because we have positive ions and negative delocalized electrons there is a strong electrostatic force of attraction between these once we can describe the structure of a metal we can use this to explain a number of its properties firstly metals are famously good conductors of electricity and the reason for this is that those delocalized electrons are able to move through the metal and carry charge as well as carrying charge the electrons can also carry thermal energy so this also makes metals really good thermal conductors metals have high melting points in order for us to explain why this is we need to have a little bit of a think about states of matter as a substance moves from a solid to a liquid and from a liquid to a gas what's actually happening is that that substance is absorbing enough energy to overcome the forces between the particles so the stronger those forces are the more energy is required and therefore the higher the melting point or the boiling point will be if we think about the forces holding a metal together the electrostatic force of attraction between the positive ions and the negative delocalized electrons is really really strong it takes a lot of energy to overcome it and that's why metals have comparatively high melting points metals are also malleable this malleability or the ability to hammer the material into a new shape is linked to those regular rows of positive ions if i hit the metal with a hammer it's possible for an entire row of irons to slide across the rows beneath them without them being affected in any way and the bonds are just going to reform between the positive ions and the electrons are where they've ended up so this allows us to hammer metals into new shapes if we compare a pure metal to an alloy we'll see it's a little bit different alloys are mixtures containing atoms that are different sizes so those nice regular rows are distorted and that means that they can't slide and this makes the alloy far less malleable and far harder instead the second type of bonding that we're going to talk about is ionic bonding which happens when electrons are transferred from a metal to a non-metal when this happens the atoms which were previously uncharged become charged particles that we call ions because now they don't have the same number of electrons and protons we show that these particles are now ions by drawing them with square brackets around them and then writing the charge in the top right hand corner the easiest way to identify how many electrons an atom is going to gain or lose is by looking at which group it's in for gcc chemistry we're only going to look at groups one two six and seven so you can simply memorize that anything in group one is going to lose one electron anything in group seven is going to gain one electron and so on if you think there's a chance you may struggle to remember this you might want to use a trick that i use with the classes i teach at school so i say to them you're going to walk into your first chemistry exam feeling really positive because we're so well prepared and you're going to feel so positive that before you've even looked at the questions on the exam paper you're going to write me a nice positive sign in the top left of the periodic table on top of group one just like that and then because you've done what i've asked you to and you feel good about it you feel even more positive so you're going to write two plus on top of group two once you've got that in it's much easier to remember that the others must be minus because they're on the other side of the periodic table so it must be the opposite we want a mirror image so we're going to have two minus on top of group six and a single minus on top of group seven and then finally i say to them you're going to draw an arrow that shows the direction that you've done this in so left to right and that arrow there represents the direction of electron transfer so if i looked at the first example that we had here with sodium and chlorine rather than going to the trouble of drawing the outer shell i could just look at my periodic table and say well here's sodium in group one so that's going to be a single plus and here's chlorine in group seven so that's going to be a single minus you'll then need to think about how many of each atom you need so for instance magnesium is in group two it has two electrons in its outer shell and it needs to lose both of them but chlorine is in group seven and it only needs to gain one electron so in order for magnesium to react with chlorine i'm going to need two chlorine atoms for every one magnesium atom although in those previous diagrams i've only shown you a couple of atoms at a time in reality when an ionic compound forms thousands of ions are simultaneously made and these join together to form what we call a giant ionic lattice this is made up of positive and negatively charged ions in a constant ratio so it could be one to one it could be two to one but in any part of that lattice we're going to have the same ratio now between these positive and negative ions there is a strong electrostatic force of attraction and it's acting in all directions so this lattice is a 3d structure these ionic compounds are going to be solids at room temperature because of that strong electrostatic force of attraction it needs a lot of energy to overcome it and so it needs a lot of energy to break those bonds and melt the substance ionic compounds can conduct electricity provided you either melt them or dissolve them in water because they are made of charged particles but in order for a current to flow those charged particles must be free to move and so this strong electrostatic force of attraction must be overcome you could also be asked to evaluate how good a particular diagram or model is at representing ionic bonding neither one of these diagrams adequately shows you that there are thousands of ions involved but one advantage of the one on the right is that it is three-dimensional rather than 2d however a problem with it is that it makes it look like the ionic bonds are physical things you could reach out and touch rather than just being a force pulling the ions together the third type of bonding that you need to know about is covalent bonding and remember this is going to happen between non-metal atoms when they share pairs of electrons now that could be one pair as in a single covalent bond two pairs in a double covalent bond or even three pairs in a triple bond as we see in nitrogen but the number of electrons that are shared between the two atoms is always going to be an even number and that's important to remember when you're drawing these you need to be able to discuss small covalent molecules specifically eight named examples that we'll look at in a second and also giant covalent structures you should be confident comparing diamond with graphite graphite with graphene and also talking about silica and fullerenes small covalent molecules or simple covalent molecules as you might hear them called are structures that contain a few atoms covalently bonded together and there are eight named examples in your gcc specification that you're expected to be able to draw without any further assistance or guidance we've got hydrogen chlorine hydrogen chloride methane ammonia water oxygen and nitrogen the dots and crosses all represent electrons they're just showing that different electrons have come from different atoms but if you did want to draw them all as crosses in the exam that would absolutely be fine it's important to make sure that your electrons are all being shared between the atoms so i would always advise you to draw them within that kind of venn diagram overlap rather than on the lines because that way we definitely know that they're being shared which is the most important thing about covalent bonding when you're drawing these you should definitely remember that oxygen contains a double bond and nitrogen contains a triple bond another thing to be aware of is that elsewhere in the gcc you need to know about unit 7 in general and you need to know that all of unit 7 bond and react in the same way so even though you've only been given chlorine to draw here in the past the exams have included questions about drawing fluorine or iodine or other group 7 elements because you're supposed to know that they're going to bond in exactly the same way as chlorine so if you can draw chlorine you can draw fluorine or iodine or whatever the substances that contain these small covalent molecules are usually gases or liquids and they have low melting points and boiling points remember the melting point or boiling point of a substance is linked to the amount of energy it takes to overcome the forces between the particles so the reason these substances have low melting points and low boiling points is because the force between the molecules is not very strong now the crucial thing here that lots of people get confused about is that the force between the molecules is not a covalent bond the covalent bonds inside each molecule are very very strong and they do not break what breaks is the weak intermolecular force between the molecules which here i've shown with these green dashed lines as we saw while looking at group seven the larger a molecule is the stronger those weak intermolecular forces are and therefore the higher the melting and the boiling points are when the molecules get big enough even covalently bonded substances can be solid and this is what we see with polymers including many plastics polymers are very long chains of repeating units called monomers and the monomers are joined together by strong covalent bonds there are still weakened molecular forces between the polymer strands and because these strands may be thousands and thousands of atoms long the forces are comparatively strong and so this means that polymers are usually solids at room temperature finally substances made of small covalent molecules can't conduct electricity because they don't have any overall charged particles like delocalized electrons or ions and the molecules themselves don't have an overall electrical charge in contrast those small molecular substances giant covalent structures often contain thousands of atoms and whereas in the simple molecular substance we had a little cluster of atoms with a couple of covalent bonds and then a weak intermolecular force to the next molecule in a giant covalent structure we have hundreds or thousands of atoms joined by strong covalent bonds and that makes these substances solids at room temperature with high melting points because those strong covalent bonds are well really strong and so they take a lot of energy to overcome there are quite a few named examples of these as well and you need to be able to discuss and compare them we start with diamond which is made out of carbon atoms and in diamond every carbon atom makes four strong covalent bonds to four other carbon atoms this makes it very very hard it also has a really high melting point because it takes so much energy to overcome those strong covalent bonds now compared to diamond graphite is also made of carbon so we would expect it to have quite similar properties but it doesn't because it's bonded in a different way whereas in diamond every carbon atom makes four strong covalent bonds in graphite each carbon atom only makes three strong covalent bonds and so that leaves one electron that's not part of a covalent bond and that electron is free to move and carry charge and this is why graphite can conduct electricity a lot like a metal the atoms in graphite are bonded into these sheets made of hexagons and these sheets are able to slide over each other because there aren't bonds from one sheet to the sheet below it and this makes graphite really slippery and that makes it quite useful as a lubricant also it's possible for us to remove a single layer of that graphite and when we do we call this graphene and graphene is a really useful material because it's able to conduct electricity just like graphite does but also it's really really thin and it can be bent and made into wearable electronics and all sorts of other exciting stuff you should also be able to talk about silica or silicon dioxide which is not made from carbon it's made from silicon and oxygen but it has properties very similar to diamond in that it has a very high melting point and it's very hard and quite strong it's often used for things like lining the walls of furnaces because it's very high melting point means that it won't just melt and fall off the walls and that high melting point just like in diamond is because there are so many strong covalent bonds now i guess it's up for debate whether we consider fullerenes to be giant covalent structures or not because they're not really very giant they often only contain about 60 atoms but they represent another way in which carbon can bond these are hollow balls or tubes and they're usually based around hexagons although also sometimes rings of five carbons or seven carbons the first one to be discovered and therefore the most famous one is buckminster fullerene which contains 60 carbon atoms fullerenes can also include nanotubes which as the name suggests are very small tubes and these can be really useful because they have a very high length to diameter ratio and also a very high strength to weight ratio and this makes them really useful in lots of nanotechnology electronics and also material science fullerenes are a type of nanoparticle so we move seamlessly into this topic but if you are doing combined science this is triple only so you can skip ahead slightly the exam would define nanoparticles as being one to a hundred nanometers or a few hundred atoms although we do also talk about fullerenes which are less than 100 atoms so do without what you will particles can be defined according to their size so nanoparticles are smaller than fine particles which are in turn smaller than coarse particles fine particles are defined as being between 100 and 2500 nanometers across and then coarse particles are from 2500 nanometers to 10 000 nanometers and they're also called dust and that is a technical term it's not just us saying oh it's a bit dusty in here all of these very small particles are advantageous because they have a really high surface area to volume ratio so if you imagine that you have a box and you cut the length and the width and the depth all in half then you would double the surface area of the volume that you have there or if you cut each side into 10 then you would multiply by 10 the surface area having a really high surface area is really useful because it means that often you can use a lot less of the substance and it makes it a lot cheaper sometimes you'll find that nanoparticles actually have completely different properties compared to the same substance when it's found in bulk these nanoparticles have lots of uses in computers catalysts coatings constructions cosmetics like sun cream and deodorant and also making highly selective sensors now we move on to unit 3 which is the quantitative chemistry unit and we start out by talking about conservation of mass conservation you know means that something cannot be created or destroyed so we have conservation of momentum and conservation of energy and physics and here we have conservation of mass or matter so what this means is that during a chemical reaction no atoms can be made or destroyed and this means that when we represent a chemical reaction with a symbol equation we must have the exact same atoms on the left hand side as the right hand side and that's why symbol equations need to balance this also means if i know the mass of the reactants for a reaction i also know what the mass of the products will be because they have to be exactly the same i'm not making or losing any atoms so i can't make or lose any mass this can get a little bit complicated when we start looking at reactions that involve gases where the mass may appear to change here's an example say i take a six gram strip of magnesium ribbon and i've put it on some scales and i know that its mass is six grams i then heat it up and i let it react with some oxygen and then when i weigh it again i have 10 grams of magnesium oxide now it might look as if that reaction has gained mass and we've just said that that can't happen but what actually happened is that four grams of oxygen that previously was floating around in the atmosphere and wasn't actually pushing down on the weighing scales is now bonded to the magnesium and so its mass is now being accounted for it's not that we've got any new atoms it's just that now they're actually interacting with weighing scale and this is what you will see where we have a reactant as a gas the reaction will appear to increase in mass likewise if we have a reaction where a gas is a product it will appear to lose mass so say i take five grams of calcium carbonate and i heat it up so that it thermally decomposes it will break down to make 2.8 grams of calcium oxide which is what i'm left with and can weigh and then 2.2 grams of carbon dioxide which floats off into the atmosphere and is no longer pushing down on my weighing scales so even though the same total mass of products exists it appears as if my reaction is losing mass next you need to be able to calculate the relative formula mass or mr of either a compound or a complex molecule this is done by adding together the relative atomic masses of every atom in that substance so for instance if we're looking at sodium oxide i can see from the formula here that it contains two sodium atoms and one oxygen atom and therefore i'm going to add together 2 lots of 23 because that's the mass of sodium and 1 lot of 16 because that's the mass of oxygen to get a total of 62 grams per mole having just said that the units for relative formula mass are grams per mole if you're taking the higher tier you need to understand what a mole is so the idea here is that one atom would be far too small for us to measure the mass of so rather than just measuring one atom on its own we measure a kind of multi-pack and that multi-pack is called a mole so when we say that the relative atomic mass of sodium is 23 what we mean is that one mole weighs 23 grams and that's compared to a gold standard which is the carbon-12 isotope now this mole is just a big number in the same way that you understand that the word million means a one with six zeros after it when we talk about a mole we mean six point naught two times ten to the twenty three and this is sometimes referred to as avogadro's constant or avogadro's number so if one mole contains 6.02 times 10 to the 23 atoms or molecules or ions or whatever the particle is that i'm looking at then i can therefore say that half a mole of a substance contains half that many particles and two moles of substance contains double that number of particles so what this means is that if i look at the periodic table and i look at the relative atomic mass of any element i can say that that many grams of that element will contain the same number of particles so 12 grams of carbon has the same number of atoms as one gram of hydrogen or 16 grams of oxygen once i know that the mass of one mole of sodium is 23 i can use this information to start to work out how many particles there are in other masses so for instance 10 moles of sodium would weigh 230 grams and i might want to work out how many moles of sodium there are within 483 grams the formula to work this out is going to be mass is mr mole strictly speaking here if i'm talking about an atom i should be using relative atomic mass rather than relative formula mass but functionally they're the same and i find masses mr mole just trips off the tongue in the way that masses are mole doesn't so if i rearrange this formula to make moles i have mole is mass divided by mr so to work out the number of moles in 483 grams of sodium i do 483 divided by 23 to get 21 moles we can start to put this together with symbol equations to do more complicated calculations called theoretical or predicted yield a theoretical yield calculation is basically asking you how much product can you make given a certain amount of reactant if we think about this in non-chemistry terms imagine that you've gone to the supermarket and you can buy apples for three pounds for a dozen and you want to make some apple pie and you know that it takes half a dozen apples to make a pie if you spend 12 pounds on apples how much apple pie can you make well the first thing you'd want to do is to work out how many apples you can buy for that money and then you'd want to work out how much pie you can make using those apples if i divide the total amount of money i have by the cost of one multi-pack i can find out that i can buy four multi-packs four dozen apples and then given that i know that half a dozen apples makes one pie i can see that i'm multiplying by two so i take my four dozen and i multiply by two to make eight pies i can go through the same process now but using proper chemistry terms instead so rather than looking up the price of a multi-pack i'm going to work out what the relative formula mass is and rather than working out how many dozen apples i can buy i'm going to work out how many moles that corresponds to so in this question i want to know how much aluminium can i make given 510 grams of aluminium oxide my first step is to work out the relative formula mass which i do by adding together the relative atomic masses of all of the atoms in aluminium oxide to give me an answer of 102. now i use that together with the mass to work out how many moles i have 510 divided by 102 is 5 moles now i look at the symbol equation to work out how many moles of aluminium oxide i can make i can see in the symbol equation that for every one aluminium oxide i have two aluminiums so that means i'm going to make 10 moles of aluminium and then finally i need to convert this to a mass so i'm going to take the relative atomic mass of aluminium which is 27 and multiply these together to get a final answer of 270 grams solutions are made up of a soluble substance called a solute dissolved in a liquid called a solvent concentration is a measure of how many particles there are in a set volume of that solution and we can calculate it by dividing the mass of the solute by the volume of the solvent this is actually a really easy formula to remember because the units are actually going to give you what you need to know you can see that the units for concentration are grams divided by decimeters cubed and we work it out by dividing the grams by the decimeter cubed the thing that you're most likely to get confused with here is that it's possible that they won't give you the mass or the volume using the correct units so you need to remember that there are a thousand grams in one kilogram and a thousand centimeters cubed in one decimeter cubed because you may need to convert between these if you're taking the higher tier you'll also need to be able to complete concentration calculations using moles per decimeter cubed rather than just grams per decimals cubed in every chemical reaction there is one reactant that is going to run out first and we call this the limiting reactant whereas the one that there is too much of is referred to as the excess this may not be as simple as working out the number of moles of each one you may need to also look at the symbol equation to work out the ratio that they should be reacting in so for instance two moles of magnesium react with one mole of oxygen gas to make magnesium oxide so if i have three moles of magnesium and two moles of oxygen it's actually the magnesium that is limiting because according to this symbol equation i need twice as much magnesium and even though i have more magnesium i don't have twice as much the next three calculations are all for the triple scientists so if you're doing combined science you can skip ahead to chapter four percentage yield is a measure of how successful your reaction has been we compare the amount of product you did make with the amount of products you theoretically could have made and express this as a percentage it will never be 100 and this can be for a number of reasons such as the reaction being reversible the reaction not finishing the reactants having side reactions with other things like oxygen from the atmosphere and also just the fact that we often lose some product during extraction atom economy is a theoretical calculation that we work out by comparing the mass of the useful products to the mass of everything including the waste products so if you go back to our example of making apple pie we could have 10 kilograms of ingredients but only make nine kilograms of pie because we had a lot of wastage so our atom economy would only be 90 when describing the method for the titration required practical you need to be confident naming equipment such as the buret which contains the solution you do know the concentration of the pipette which is used to measure a set volume of your unknown concentration solution and the conical flask which is used that the solution doesn't splash when you're swirling the conical flask should be on a white tile so you can clearly see the colour change and you should talk about measuring volumes at eye level looking at the meniscus when you write your method it needs to make sense and actually work so make sure that you've remembered to add an indicator so that you can identify the end point when the color changes make sure that you've talked about actually adding the two solutions to each other swirling to keep the mixture homogenous recording the volume when you reach the end point and also repeating the experiment until you have concordant data to complete titration calculations you need to work out the moles of one of the reactants using concentration is moles divided by volume then use the symbol equation to work out the moles of the other reactant at the end point and then use moles and volume together to work out the concentration of the missing reactant the fourth unit is chemical changes which starts out looking at the reactivity of metals you need to be able to describe five general reactions and use these to complete word equations and also you need to be able to talk about using your observations that means things you can see to put metals in order of reactivity the first reaction that you need to know about is that metals react with oxygen to make metal oxides so this could be as simple as a word equation where you're asked copper plus oxygen reacts to make and you just need to say copper oxide metals react with water to make metal hydroxides and hydrogen so again you could be given the name of a metal and asked to complete a word equation lithium plus water reacts to form lithium hydroxide and hydrogen by observing these reactions we can put metals into an order of reactivity so if i compared these two tubes i can observe that the potassium is producing far more bubbles and this tells me that potassium is more reactive than copper you should know that metal hydroxides are alkalis and so you can test for them using universal indicator which will turn blue and to test for hydrogen we can ignite it or set fire to it and it will burn rapidly with a squeaky pop sound the third reaction is metals reacting with acids to form a salt and hydrogen so again we can ignite the hydrogen to prove that it is hydrogen and for the salt we need to be able to name the particular salt that is made there are three examples of strong acids that you need to know about for gcse and each one makes a characteristic salt so if i put some tin in some hydrochloric acid i would make tin chloride or if i put some lead in sulfuric acid i would make lead sulfate or if i put some zinc in some nitric acid i would make zinc nitrate once you know that third equation the fourth one is quite easy to make a metal oxide we take a metal and add a little bit of oxygen so we're also going to need to add a little bit of oxygen to the products so you're still going to make the exact same salt that you made in the third reaction but this time rather than making hydrogen you make water because that's what you get when you add a little bit of oxygen to some hydrogen similarly to turn an oxide into a carbonate we add one carbon atom and two oxygen atoms so the products for a metal carbonate reacting with acid are exactly the same as a metal oxide but then also some carbon dioxide to test for carbon dioxide we bubble the gas through lime water which will turn cloudy we've now come to the first combined science chemistry required practical in which you need to make a sample of a pure dry soluble salt the first step is likely to be picking the reactants you're going to need to use so you need to pick the appropriate acid remembering that sulfuric acid makes sulfates hydrochloric acid makes chlorides and nitric acid makes nitrates you then need to heat up that acid in order to increase the rate of reaction you're going to add an excess of an appropriate base which means you're going to add far too much of it to make sure that all of the acid reacts and that base will either be a metal oxide or a metal carbonate you then filter this in order to remove the excess base and then finally you evaporate most but not all of the remaining water using either a bunsen burner or a hot water bath how reactive a metal is is directly linked to how easily it forms positive ions but that's not something that we can see so instead we use our observations of chemical reactions in order to place metals in order of reactivity be very careful that if the exam asks you for the observations you're making you are only talking about things that you can actually see or detect yourself the specification says that you need to know the order of reactivity of potassium sodium lithium calcium magnesium zinc iron and copper although you may have also learned some additional materials make sure that you know where carbon and hydrogen go in this list so carbon is less reactive than magnesium and hydrogen is more reactive than copper some metals like gold are so unreactive that they can be found in their elemental form in the earth's crust but most metals need to be extracted from other compounds one way of doing this is by reduction heating the metal compound up with carbon so that the carbon will remove the impurities such as oxygen this is an example of a displacement reaction and that means it will only work when carbon is more reactive than the metal so we can use reduction to extract iron but not to extract the alkali metals we also only use it in situations where the carbon doesn't react with that metal as otherwise we would just have a different compound if you're sitting the higher tier then you also need to be able to describe oxidation and reduction in terms of electron transfer and the nice way to remember this is oil rig oxidation is loss of electrons reduction is gain of electrons so if we look at a chemical equation such as magnesium reacting with hydrochloric acid then we can break this down and say that the magnesium has gone from being magnesium as an atom to magnesium as a two plus ion and this has happened by the loss of electrons so this is an example of oxidation the hydrogen has gone from being a single positively charged hydrogen ion to hydrogen as an atom and therefore it's gained an electron and this is an example of reduction you should know that acids are substances which dissolve in water to release hydrogen ions whereas alkalis dissolve in water to release hydroxide ions the ph scale is a measure of acidity which runs from 0 to 14 where 0 is highly acidic 14 is highly alkaline and 7 is the neutral point in the middle you should know that acids and alkalis react together to form salts and water and this can be expressed by the ionic equation where the hydrogen ion reacts with a hydroxide ion to make water the salt isn't included because it's different for every single different pair of acids and alkalis if you're taking the higher tier you also need to be able to describe strong and weak acids strong acids such as hydrochloric acid sulfuric acid and nitric acid are completely ionized when they dissolve in water which means that they all give up their hydrogen ions and they have a very low ph whereas weak acids like citric ethanoic and carbonic acid are only partially ionized and so this means that for the same concentration they will have a higher ph finally in unit 4 we have electrolysis and this is a pretty big topic so if you're unsure i would really recommend looking up the separate electrolysis video electrolysis means splitting ionic compounds called electrolytes into their elements using electricity the electrolyte must be either melted or dissolved in order to break down the ionic lattice and allow the ions to move we call the positive ions cations and the negative ions and ions these ions move in response to charge with the positive cations being attracted to the negative cathode and the negative anions being attracted to the positive anode because opposites attract when the ions reach the electrode electron transfer takes place and the ions are discharged so they stop being charged and they're turned back into atoms if you're sitting higher tier then you need to be able to describe these processes using half equations like this one so since aluminium has a three plus charge i know that it will need to pick up three electrons in order to discharge and because this is the gain of electrons i can describe as reduction anion discharge is slightly more complicated because most non-metals aren't going to go around as single atoms they're going to go around as molecules so because oxygen goes around as a divalent molecule i'm going to need to discharge two oxide ions rather than just one since each oxide ion has a two minus charge it needs to lose two electrons and that means that in total four electrons are going to be lost and since this is electron loss we're going to call it oxidation you should also be able to describe the extraction of aluminium as an example of electrolysis so you need to know that it's extracted from a mixture of aluminium oxide and cryolite which is added in order to disrupt the ionic lattice and help it to melt at a lower temperature thereby saving energy and saving money you should know that this is a really expensive process because of the amount of energy that's required to melt that lattice because of the electricity requirement and also because the electrodes keep wearing away and needing replacing the electrodes are made from graphite because this is relatively unreactive quite cheap and a really good conductor of electricity but at the high temperature that's needed to melt the aluminium oxide even unreactive graphite will start to react with the oxygen that's being produced at the positive electrode and so over time the graphite electrodes just wear away to make carbon dioxide and they have to be replaced finally for unit 4 you need to discuss the electrolysis of substances that have been dissolved to make solutions rather than melted as well as the two ions from the ionic compound these electrolytes will contain hydrogen ions and hydroxide ions so each electrode will have two different ions competing for discharge the rule is that at the negative electrode the more reactive ion will stay in solution and the less reactive iron will be discharged so that means you will either make hydrogen or a jewelry metal copper silver gold or platinum those are your only options then at the positive electrode if you have a halide ion like chloride iron you'll discharge a halogen and if there's no halide then you will discharge the hydroxide ions to make oxygen if you're sitting the higher tier then again you need to be able to use half equations to describe this and i would particularly recommend memorizing the one for hydroxide ions because it is really tricky to figure out under pressure the fifth and final unit in aqua gcc chemistry paper one is energy transfers and we start out by saying that energy is conserved which means it can't be created or destroyed we can split chemical reactions into two types and the first type are exothermic reactions which are reactions that transfer energy to the surroundings and this will usually mean that the surroundings get heated up these include combustion reactions many oxidation reactions and neutralization exthemic reactions can be used for making self-heating cans and also hand warmers endothermic reactions absorb energy from the surroundings and so for most of the time this means that they will cool the surroundings down examples include thermal decomposition the reaction of citric acid with sodium hydrogen carbonate and also photosynthesis which of course absorbs light energy they're used for making those sports cool packs that you can sort of snap to make them get cold rather than needing to put them in the freezer there is a required practical as part of unit 5 and in it we investigate the variables that affect temperature changes in reacting solutions there isn't one named chemical reaction that everybody has done for this so you might have done different investigations like adding acids to alkalis or adding metals to a solution like copper sulfate and it's entirely possible that the exam board may name you a reaction you haven't done and ask you to answer questions in the same style if you're writing a method regardless of what the reaction is you're going to want to be measuring the temperature change so that you can identify if the temperature goes up then the reaction is exothermic and if the temperature goes down then it's endothermic you'd probably want to write in your method about completing the reaction in an insulated vessel like a polystyrene cup and using a lid to prevent heat losses to the environment and you might want to use a digital thermometer because this will give you more precise results as with any of these investigations once you've decided what your independent variable is going to be the thing you're going to change you need to make sure that you're controlling other variables that could affect the outcome of the experiment so for instance if you're adding metal to copper sulfate solution you would want to use the same mass of metal each time if you were changing the type of metal that you were adding controlling these things is going to increase the validity of your experiment you don't want to say making a fair test because this won't get you credit even though it basically means the same thing we can also represent the energy changes that take place in a chemical reaction using a reaction profile for each one of these diagrams you can see that there are horizontal lines representing the amount of energy stored in the chemical bonds of the reactants and the products if you look at the diagram on the left you'll see that the products have less energy than the reactants now we know that we can't have lost any energy somewhere so that energy must have been given out to the surroundings heating them up so therefore this diagram on the left is an exothermic reaction whereas the diagram on the right where the products have more energy means that the products must have taken in some energy from the surroundings and therefore this is an endothermic reaction for each reaction you can see i've drawn a pink arrow that goes from the height of the reactants up to the highest point which we call the transition state each of these arrows represents the activation energy which is the minimum amount of energy required for this reaction to proceed in a bond energy calculation you may be asked to work out the overall energy change that takes place as part of a reaction for this you need to know that breaking bonds is going to take in energy and making bonds is going to release energy whenever you approach one of these questions i would recommend that you start by drawing the actual number of molecules that are involved because this will stop you from making silly mistakes where you forget to double something once you have these molecules you need to add up the numbers that represent all of the bonds on the left hand side and all of the bonds on the right hand side once i have these numbers i do a simple subtraction by writing a minus sign between the two of these the energy change is calculated by doing energy taken in minus energy released so my number on the left take away my number on the right gives me my overall energy change for the reaction which in this instance is -486 a negative number always represents an exothermic reaction because it tells me that the chemical energy stored in the products is less than that in the reactants this final section is only for the triple scientists so if you're doing the combined science exams congratulations you are done cells contain chemicals which react to produce electricity and the voltage that a particular cell makes is dependent on a number of factors including the type of electrode and the electrolyte that it submerged in you can make a simple cell by connecting two different metal electrodes with an electrolyte and this even works if you use a fruit and you can use the citric acid from the fruit as the electrolyte when two or more cells are connected together in series we call this a battery and it provides us with a greater voltage in non-rechargeable cells and batteries the chemical reactions will stop when one of the reactants have been used up and alkaline batteries are an example of a non-rechargeable battery in contrast to this rechargeable batteries and cells can be recharged because the chemical reactions are reversed when an external electrical current is applied you may also be asked to compare and contrast chemical cells and batteries with fuel cells in which there's an external continuous supply of a fuel such as hydrogen in a hydrogen fuel cell the overall reaction is that hydrogen reacts with oxygen to make water but rather than a traditional chemical reaction where the hydrogen and oxygen actually touch and there's a huge loss of heat energy instead the hydrogen is turned into hydrogen ions within the electrolyte and the oxygen reacts with some water to make hydroxide ions and then these hydrogen and hydroxide ions come together to make water this is a much much more efficient process and doesn't involve a lot of loss of energy as heat hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries and could be much better for the environment however it's worth being aware that at the moment there are only two real ways of getting the hydrogen to use as a fuel and neither of them is particularly good for the environment either you electrolyze water which needs a huge amount of energy or you react methane with steam which is using up a fossil fuel one of the biggest stumbling blocks of fuel cells is knowing the names of the electrodes because often people have been taught a method for electrolysis which then doesn't work for fuel cells it's actually reversed a better way of remembering how to name your electrodes is that reduction occurs at the cathode and oxidation occurs at the anode and this method works for electrolysis and for fuel cells if you have learned panic or a similar method then you need to remember that the positive negative is reversed from electrolysis to fuel cells you also need to know the half equations that happen at each electrode so at the negative anode we're going to see hydrogen splitting apart to make four hydrogen ions and four electrons and then at the positive cathode you're going to see oxygen reacting with water and four electrons to make four hydroxide ions these will later go on to join up with those four hydrogen ions to make water molecules finally we're going to think about the advantages and disadvantages of each of fuel cells and chemical cells fuel cells by definition run as a continuous process you keep giving them more and more fuel and they just keep running and there's never any reduction in efficiency in contrast to that a cell or a battery is based on a chemical reaction where the chemicals are going to run out and then the battery will need recharging also each time that you recharge it it holds slightly less charge and you'll have seen this if you've had a phone for any length of time and gradually your battery life gets less and less for a hydrogen fuel cell the only waste product is water but the reactant the hydrogen is highly explosive and quite difficult to store rechargeable cells often have waste products that are very toxic and need special disposal hydrogen fuel cells do have a relatively low potential difference and that means you can't use one cell in isolation you need to wire several together in series to make a battery rechargeable cells have a larger potential difference and that means we often can use them as single cells although you can still wire them together to make a battery if you want to that's it for aqa gcc chemistry or combined science paper one i hope that you found this a useful summary of all of the content that is going to come up in your exam good luck and don't 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