Teacher EXPLAINS Chemistry Paper 2 2023 GCSE Science AQA

Hey guys, welcome back to another GCSE Science video. This time we're doing Chemistry, Paper 2, 2023. I'm going to go through every single question, hopefully I can explain it in a pretty good way so that you understand the actual content as well as how to actually get the marks in the exam. Once again, before we get started, if you ever found any of these videos useful please do consider subscribing as it does help me a lot, and in the description there is a email address where you can send me any questions you have about tutoring or specific questions about science, maths, or whatever you want to do, maybe options for A-levels as well. But let's just jump straight into this. So, Chemistry, Paper 2. You might be wondering why I haven't done Paper 1 first, it's mainly because I'm doing it in order of what people have been asking me for, so if you do have any suggestions then do put it in the comments, like any of the comments that have a suggestion that you like, because that's essentially how I've done this. A lot of people have asked for Paper 2, so that's where we're starting. So let's jump in with Question 1. For Question 1 we have the combustion of fuels is a source of atmospheric pollutants. Methane is a fuel. Balance the equation for the combustion. So I do get asked a lot of questions about balancing equations, how do I do it? Well, the way I do it is, honestly I keep it as simple as humanly possible, and normally these questions can be simple, there are some awkward ones with like iron oxide for example that can get a bit weird, but I just go through each and every element and make sure it's balanced. So for example here, if I start with the C's, how many C's do I have on the left side of this equation? I have one C, right? So we have one C. And on the other side, how many do I have? I have one C. So in other words, those C's are balanced at the moment. I have to check this after every single change I make. Now what about hydrogens? Well I have four hydrogens here, right? H4 means there's four hydrogens. It's methane, so that also makes sense. And on the other side I have just H2, so that means I have two hydrogens. So how can I change two into four? I can just shove a two in front of here, and that gives me four hydrogens and four hydrogens. So now they are balanced. Perfect. Then I go to the oxygen. So how many oxygens do I have? I have two oxygens on the left at the moment, and that looks like 20 now, lol. And on the other side I have two plus two times one, so I have four oxygens on the right. So what I'm going to do, I'm going to add a two in front of here to now make them all four. And that's it. Now I can double check again. I still have one carbon on each side, I have four oxygens on each side, I have four hydrogens on each side, and that's it. You may have to change multiple numbers, that's completely fine, but just make the letters on both sides the same. For part two, many fuels are mixtures. Petrol and diesel are mixtures of hydrocarbons. Table one shows properties of petrol and diesel, and so if we just read through this nice and quickly, it says the range of number of carbon atoms in a hydrogen carbon molecule, for petrol it's 4 to 12, diesel is 12 to 20. Okay, there's a difference there. The range of boiling points, petrol seems to have a lower boiling point by far than diesel. So it says compare the properties of petrol and diesel, use table one. So when it says use table one, what does that mean? It means that you have to use the table, right? So you're not just comparing petrol and diesel in the real world, like, I don't know, it's price or something else, you want to try and stick to the table itself. So what's the first thing we can say? Well, the first thing we can say is, and if you notice by the way, two marks and there's two rows, because there's two properties, right? So we can compare the number of carbon atoms, then compare the boiling points. Now you don't have to actually know anything about petrol and diesel, this is more, I mean, they've given you the information, right? This is comprehension. So I can say that petrol has fewer carbon atoms, or fewer range of carbon atoms, than diesel. So I will write that, so petrol molecules, here, when I say petrol molecules, I mean the molecules in petrol, not a petrol molecule, because that doesn't exist, have a smaller number of carbon atoms. You could also say the complete opposite, so you could also say the molecules in diesel have a greater number of carbon atoms and get the same thing. And again we can say that, well, what's the other thing it shows? It shows that petrol has a lower boiling point. So petrol has a lower boiling point. That's it. I should probably say then diesel, you know, to be really, really specific. That's it, two marks and you're basically just reading, not bad. So for part three, the gas is released when a fuel is burned in a car engine, or in car engines, I can't read myself, may include oxides of nitrogen, carbon monoxide, water vapour. Which chemical element do these all contain? Now if you weren't sure what the chemical formulas were for this, that's fine, because we can also use a little bit of logic, you know. So for example, with carbon, well this just says oxides of nitrogen, and this is water, and I'm pretty sure water doesn't have carbon in it, it's H2O, so I know it's not this one. What about hydrogen? Well water does have hydrogen, but does carbon monoxide and oxides of nitrogen? Doesn't sound like it. What about nitrogen? Well the first one obviously has nitrogen in it, but water doesn't have nitrogen in it, and neither does carbon monoxide. So oxygen is the only possibility, and does it make sense? Well water vapour is H2O, carbon monoxide is CO, oxides of nitrogen, it can be, it's going to be NO something, right? So all of those have oxygen in them, so that must be the only answer, right? For part four, when diesel burns in car engines, oxides of nitrogen are produced, so N and O. Where does the nitrogen come from? This is a good point, because again, in car engines, it is a mostly sealed compartment, we burn with oxygen, but where does the oxygen come from in a car engine? It comes from the air. What else is in the air? Nitrogen. So basically it just comes from the air. So when car engines take in air, it also has things like nitrogen and all these other elements as well. So that's where the nitrogen, which makes oxides of nitrogen, comes from. When diesel burns, particulates may be produced. Now a lot of students aren't too sure about what a particulate is, and that is relevant to this here. So what environmental effect do particulates from burning diesel cause? Now the key thing is, particulates are not greenhouse gases, okay? They're not greenhouse gases, so it's not global warming. Particulates always produce global dimming. The way I like to think of it, have you ever been in a really dusty room, and then you'll see sunlight come through a window, and you see those little dots in the air floating around? That's essentially what a particulate is, and what it does is it dims the light. So if you're in a really dusty room, it's less bright than if you're in a completely clean room. For a similar reason, in fact for the same reason, that particulates cause global dimming. So if you imagine that this is the earth, this is the atmosphere around it, that she's a- she's black, why not? That's a bit disturbing, but whatever. When light comes in, the reason why it's bright is because the light passes through, and it hits the earth, and reflects off of things, and all of that fun stuff. But when we have particulates in the air, what do you think happens? Well instead, some of this light is being blocked, and it gets hit, it reflects off of the particulates themselves. So that means not as much light is actually reaching the earth, which means it's less bright, and this is an actual measurable effect. It's significant. It's only a few percentage points, or fractions of a percentage point, but that is relevant when you talk to the whole earth. And again, the best way to think of this is when you're in a dusty room, and you look at the light, you look at it side on, and you see those dust mites floating around in the air. It's a similar concept to that. Particulates are little fractions of stuff. So it could be dust, kind of, and other kind of unburnt things. So they aren't gases, necessarily. They're solids that can float around in the air. It's like when you, you know, throw sand in the air, for example. Carbon monoxide may be produced when diesel burns. Give one reason why carbon monoxide is difficult to detect. There's two reasons. The first is, and the first kind of mistake I see students write, is they say because it's a gas. Gases are not hard to detect, necessarily, because there are gases that have a colour to them, like chlorine gas. There are also gases that have an odour to them. You can smell them, which means you can detect them. Simply put, carbon monoxide, by itself, is colourless, so you can't see it, and it's odourless, which means you can't smell it. By the way, you only need to give one of these two reasons. So you might be thinking, well, hang on a minute, I've smelt carbon monoxide before, either when I've been in school, or when I've been at home, or something like that. That's because the carbon, the gas that gets provided to us in our homes, specifically has smelling agents added to it, so we can actually smell it and detect it. We also have carbon monoxide detectors, which is a bit different, but that's why it's difficult to detect. It's hard for you to know if you're breathing in carbon monoxide. This is a bit of an interesting question. It says, explain why water vapour and not liquid water is produced when diesel burns. Well, if we go all the way back up to the table, when you set fire to diesel, right, or petrol, what do you think happens? Well, it's obviously going to be very hot, isn't it? So that's the first thing you can say, is that diesel burns at a high temperature. Well, you know, let's think about that for a second. Well, the boiling point of water is relatively low, so it burns above the boiling point of water. Now, here's where some students get a bit confused. First of all, is the chain of logic. Now, now that I've said that to you, you're probably thinking, well, yeah, obviously. But would you write that in the exam? The second thing that people get confused on is this second bit. That's a bit of a bold assumption, right? Saying that it burns above the boiling point of water. How do I know? Well, I know because it's water vapour. So you can use the question to answer the question, right? Because since there is water vapour present, then the water must have boiled, because that's how you get water vapour, you boil water. Which means it must be burning above the boiling point of water, because the water has boiled. You know, I know it's circular in the logic, but that's the way I think of it, is if they're saying water vapour is present, it means it must have been boiled. Why is it boiled? Well, diesel must burn really hot then, right? It must produce a lot of heat, because it's produced liquid, sorry, gaseous water. That's the way I think of it. And here they're talking about a very, very specific pollutant. So sulphur is a common impurity in diesel. Why is this an environmental problem? Well, hopefully you remember your pollutants. So sulphur, which pollutant is a product of that? Sulphur dioxide. So sulphur dioxide, you get a mark for mentioning it, is produced. And why is sulphur dioxide bad? Well, it's acidic, so it causes acid rain. So, so far we've got the chain of reasoning, right? Sulphur is present, which produces sulphur dioxide, which produces acid rain. Now we're saying, why is that an environmental problem? Why do we care about acid rain? Why is that bad? Well, now you can talk about about a million different things. So you can say that it damages buildings, or statues, or bridges, or whatever. It damages infrastructure. You could also say that it damages trees and plants. You could say it's harmful to fish and aquatic life. You can say it causes respiratory problems. You can say it damages the soil. There's a million things you can say, but because it says environmental problem, you have to link it to an actual environmental problem. Either breathing problems in humans, damaging plants, damaging bridges, and things like, well, I keep saying bridges, buildings is what I'm thinking of. Any of those things would get you the third mark. But, again, you need the chain of reasoning. What does sulphur produce? It produces sulphur dioxide. Well, what does that produce? Acid rain. Why is that a problem? Because it causes one of those ten different things that I mentioned. And that's the end of question one, the environment stuff. Chromatography is used to separate mixtures. Chromatography involves a mobile phase and one other phase. Now, it says, what is the other phase in chromatography? This, again, is a bit of memory, but you could also be a little cheeky and try and work it out. Mobile, what does that mean? It means movement, right? So the other phase, you might think, would be a stationary phase. So it's moving and not moving. There's only two phases. And that is actually the right answer. Okay, so what if you weren't sure? Well, mobile and moving mean the same thing. So we can definitely cross that out and increase our odds from one in four to one in three, which is a lot better. The other two, recycled and viscous, they sound plausible, even if they're wrong. So that might be a bit harder to think about. So yeah, you can definitely eliminate moving because it just means the same as mobile and there's no reason for there to be two phases named the same thing. We'll do parts two and three at the same time. So part two says, why do the substances in the mixture separate in the mobile phase? And there's three different answers that you can give. The first one is the idea that they move at different speeds. So the ones that move faster will go further up the paper, logically, right? And the ones that move slower will not be able to move as far up the paper. Another thing is, you can say that they have different attractions to the paper. Different attractions to the paper. And lastly, and I'd say this is the real reason, is that they have different solubilities. I think that's a very good point. Now, in terms of how many spots will be produced on the chromatogram of a pure compound. If it's a pure compound, it means that there's only one compound in there. There's only one thing. And bearing in mind, the substances in the mixture separate, right? So if you have three different things, you should have three different spots. Two different things, two different spots. So if you only have one thing, one compound, there should be one spot. Because it can't separate. The reason why they've asked you this question is because, and this is a really key thing, chromatography does not separate compounds into different atoms and elements. It separates different compounds completely. Not compounds into different elements. That's the really key thing. Okay, so calculating RF values for the blue colour. So whenever you calculate RF values, you're comparing the compound, in this case a blue colour, to the solvent itself. And the RF value is always the compound, right? So the colour in this case, divided by the solvent measurement. So 4.77 over 5.30. And that's just, again, hopefully a fairly easy thing for you to do. 4.77 divided by 5.30 gives me 9 over 10, so 0.9. There you go. Two marks, and I think that's quite nice. There are no units because this is actually a ratio, because they're both centimetres. So you don't have to worry about units, which is also nice. Okay, so for the six mark experiment. Now, in terms of where you get your marks from, they're broken up into levels. And the levels are 1, 2 and 3. Shocking, I know. I guess there's technically a fourth level, which is just 0, where you get no relevant content. So the first level, you can get 1 to 2 marks out of 6. And essentially, the experiment you've described will not lead to a valid outcome, but you have some relevant steps. So maybe it's like putting the paper into the solvent, as an example. Level 2 is, the method wouldn't necessarily lead to a valid conclusion or an outcome. However, you have most of the steps, but they aren't necessarily logically sequenced. That's the key thing, right? Like, for example, if you leave it to run and then put the paper in the solvent, it's like, what are we on about? And the level 3 is, you actually get a valid outcome and it's in order. This is why bullet pointing is really, really key. Now, it says we need to separate colours in black ink, that's our first success criteria, and then identify the colours from the RF values. You need to mention a way of identifying them. Now, that means you have to calculate the RF values and then compare them to known RF values, right? So, if we do this, what would be the first step? Well, the first step, before you do anything with the chromatography paper, is you have to draw a pencil line at the bottom to know where you actually start from, right? So, draw a pencil line at the bottom of the, let's call it what it actually is, I just didn't want to write chromatography about a million times in this question, paper. The reason why we do that is we want to see the starting point, right? It's very difficult, don't forget, this, the dot that we put on a black ink is going to move and spread out. How do we know where it starts from? You use a line because we need to measure the distance, right? If you look at the previous question, they measured the length of the paper that the dot moved. Now, what do we do? We put a black dot, so place, I'm going to say a small black dot because the smaller it is, the more precise we can be in our measurements, actually no, on the pencil line. Remember, we're trying to measure where it starts from, that's what the pencil line's for, then it's going to spread out, it's going to move, the colours are going to separate, and we're going to measure the lengths. Now, what do we do? Well, now we actually do the solvent, right? Because the solvent, as it gets absorbed up the chromatography paper, will drag the ink as well. Now, the colours that are more easily dragged, so they have either a higher solubility, for example, they will get dragged up more, or they'll move faster, whatever you want to call it. So, we're going to place the chromatography paper into the solvent. Now, when we say that, there's a few caveats here, right? You're not just dunking the whole thing, you're doing it so that the level of the solvent is below the pencil line, because again, we don't, otherwise the ink's just going to dissolve and spread out. So, let's say your paper is like this, at the moment it looks like this, right? You have this big rectangle, we have your pencil line at the bottom, and we put a dollop of black ink over here. What I would do, is I'd probably put the pencil line, sorry, the solvent up to about here, because what's going to happen is, the solvent will start to move up the paper, so it'll start to move up the paper, in fact, I can just select this, move up the paper, and when it gets to the black dot, it's going to start dragging the colours inside black, which is all of the colours, up the paper, and then you're going to get separated out into like red, blue, and then depending on their solubility, or how quickly they move up the paper, that depends on how high they are. Afterwards, we're going to get a ruler and measure from the dotted line, up to each of the colours, in order to find out what their RF values are. Hopefully that makes a bit of sense. So, back to this, but again, that's where we put it below the line, if you put it above the line, then it's just going to dissolve and spread out, it's not actually going to do anything properly. It's the dragging we care about. Then we're going to leave the, I'm going to say leave the experiment to run, there we go. So, what that means is you leave it for minutes, hours, whatever you want. Then we're going to remove the paper from the solvent and wait for it to dry. Then we're going to find the RF values by measuring, come on, the distance, oh my god, distance, of all of the dots to the start line. Lastly, compare these RF values to known RF values of colours to determine the colour. What's wrong with my hands, man? It's not even cold. So, you might have noticed that I've done seven bullet points instead of six. Honestly, the way a lot of some teachers teach it, where it's, oh, a bullet point per mark, that's a little bit outdated now because it is based more on the levels that I mentioned before. So, if you have to do seven or eight bullet points, that's fine, just make sure you put them in a logical order. So, if you notice, this is, you could follow these steps right now and do a chromatography experiment. That's why you have to do it in order. I could have done draw a pencil line, find RF values, then place a small dot, then leave the experiment to run. I couldn't do it in that order. So, do it like a list of instructions. That's also why bullet points are really, really useful. And yeah, don't forget both success criteria. So, separating the colours, that's what we did with the solvent, then identifying the colours from the RF values. That's literally my last bullet point or two. That's how you get all six marks in this question. Question three, crude oil, very fun. So, crude oil is a mixture of many different compounds. Give two reasons why it's not a formulation. So, formulation means it can be given a chemical formula. Well, the reason is that, well, first of all, there's two ways to word this. It's not useful or not designed for a purpose, doesn't have a specific purpose. So, we tend to have formulations in order to create different compounds and things like that, but those are all designed to have specific purposes, right? Crude oil is not, it's just literally how it's dug out of the ground, in the same way that you wouldn't say that wood has a formulation or something like that, or dirt or mud or something like that. The next reason, this in my opinion is a bit more important, is it doesn't have a formulation because crude oil itself can have completely different compositions. So, for example, the crude oil in the Middle East, so in like Iraq for example, is very different from the crude oil in America, which is very different from the crude oil found off the coast of Venezuela. They have different combinations and things like that, there's different qualities of crude oil, and if you don't want to go into chemical engineering in terms of oil and things like that, you'll know the different qualities of oil, okay? And there are many, many different qualities of oil. So, that's another idea, is that it's not, it has differing compositions. So, it's not always a certain percentage of methane, a certain percentage of ethane, a certain percentage of whatever, has differing compositions. Okay, part two, describe how it's separated into fractions. They've done this very nicely, because they've said fractions instead of some other word they could have used. This is using fractional distillation. Now, unfortunately, naming fractional distillation doesn't actually give you a mark, because you have to describe what it is. It says describe, it doesn't say state the process. Describe, command word, means you have to actually explain how the process is done. You have to describe how the process is done. So, what happens? Well, we basically boil crude oil. Crude oil is vaporized via heating. There is a, it's placed into a fraction, how do I wanna, fractionating column, where temperature decreases as you go up the column, as you go up the column. I'll draw this out in a second as well. In fact, no, I'll draw this out now. So, you've probably seen this before, but it looks like a big kind of tank. At the bottom, your boiled crude oil goes in, and then as you go up, there's very set levels where the temperature decreases. The reason for that is gases rise, so as you go up, eventually, the temperature gets low enough for the different compounds to begin condensing, and when they condense, they are piped off. So, they're siphoned off at different levels. So, you get right to the top, which would be kind of like the gases as well. Although, you wouldn't have gases if you have crude oil, obviously, but at the top would be the very, very low, small chain hydrocarbons all the way at the top, like things like ethanol, for example. But towards the bottom, you have the much longer chain hydrocarbons, which have a lot bigger boiling points. When they go below their boiling points, so when the temperature decreases to below their boiling points, they turn to a liquid, they condense, which means they can then be piped off to the side. So, what happens next? Since it uses the terminology fractions, I'm going to use the terminology fractions. I would say hydrocarbons, but that's fine. Different fractions have different boiling points, so the different fractions are siphoned off as you go up the column. That's it, four marks. Not too bad, four bullet points in this game. Part three. The fractions from crude oil contain alkanes. Why are alkanes cracked? Well, what is the process of cracking? If you have a really long hydrocarbon, so let's say something like this, can we just pretend that I've drawn all the hydrogens as well? We can crack it. Well, the thing about cracking is, is when you break into pieces, isn't it? So, I can break, break, break, something like that. But the question is why? Well, well, first of all, actually, we can write this. The purpose is to make smaller hydrocarbons. That's the reason, because that's what cracking is, right? When you crack something, you break it into small pieces. So, in this case, we crack it to make smaller hydrocarbons. But why? Well, you can use various different reasons, but here's the general reason. The general reason is because smaller hydrocarbons are more useful. When you get to the really fat, chunky hydrocarbons, they're basically useless. They're very thick and viscous, which means you can't use them as a fuel, because they're too thick, you can't pump them. They're basically solid at that point. They also burn at a really, really, really high temperature, which, again, isn't really that useful. And they also burn very dirtily, which, again, is not very useful, because it's very hard to combust them completely. So, effectively, the most useful hydrocarbons are the ones that burn, and burn cleanly. So, we can also use it as fuels. We could also say that they are good starting materials. So, we can also join hydrocarbons together. So, if we break them, for example, if we only had chunks of five, I could never make a hydrocarbon that's eight carbons long. But if I had chunks of two, or chunks of one, I can make far more hydrocarbons, which is very useful. Here we have a cracking equation. I'm not saying it's really good. So, cracking decane. So, we have here, and it breaks off a couple of hydrogen atoms. So, it makes decane. Describe a test to identify the gas produced in the reaction. Now, they've told you that the gas is H2, so hydrogen gas. Yeah, honestly, this is a little bit of logic. I hate using that word, because it's not even logic, sorry, it's memory. But there are two tests that are really, really, really similar. This test is the burning splints test. Now, the similar one here is the glowing splint test. What's the difference? Burning means it's on fire. Glowing means that it's really, really hot, so it isn't on fire actively. Now, why is it similar? Well, funny enough, if you put a glowing splint with hydrogen gas, you get the same result as if you put a glowing splint with oxygen. So, we can't use glowing splint, because it doesn't tell us if it's hydrogen gas or oxygen gas. It just tells us that there is hydrogen or oxygen or any other flammable gas. However, when you put a burning splint with hydrogen, it pops. So, the result would be a squeaky pop sound, which is actually almost like a mini explosion. Okay, so alkenes are produced in cracking. The general formula for the homologous series of alkenes is CnH2n, which formula represents an alkene. So, basically, we want the carbon atoms to be double that, and if you notice, a lot of these have twos. So, if I sub in two into this formula, that would be C2H2x2 is 4. So, that's that one. There you go. We can check this one if we want. So, if it's C3, 2x3 is 6. That isn't on there. Nice and easy. So, we have a evaporating basin, some water applying heat. This is the method that they used in order to basically take any dissolved substances. So, they weighed the basin, added water, weighed it with the water, heated the sample, and then weighed the basin and the contents again to see if the weight should be the same. In theory, it should be the same as the original weight, unless there's extra stuff that's been evaporated, dissolved in the liquid, which is now being left behind. The method used by the the student could make to obtain valid results. There's a couple of things. The first one is, here it says heat the water for two minutes. What if not all the water evaporates? So, we could say heat until all of the water has evaporated. Yeah, and that'll probably be the best way to do it, to be honest, because otherwise you don't know if you've got water or you've got dissolved stuff as well. This is basically a maths question. So, here's the seawater. Test 1, 2, 3, and then the mean, but test 2 is missing. So, well, with the mean, there's a couple ways to do this, but what they've done is they've added up all the three values together, and they've divided by three in order to get the mean. So, all I need to do is essentially rearrange. So, I'm going to times both sides by three to get, and I'm going to add these together, right? So, x plus 0.56, 0.3 plus 0.26 equals 0.29 times three. Let's see, 0.87, which means x is 0.87 minus 0.56, which is 0.31, and that's it. Not too bad. Now, the student concludes that distilled water is pure because the mass of dissolved solids, so the difference between those masses, was zero. As we said earlier, that's what it should be, because distilled water should have no dissolved contents, whereas seawater obviously has salt in it. But we have to describe a test. There's actually two. We could test either the boiling point or the freezing point of the water. So, we could determine boiling point. Boiling point. Why? Because it should be that pure water boils at 100 degrees Celsius. We could also do the same thing with freezing. So, if we froze the water and determined the freezing point, it should be zero degrees for pure water. Seawater does not boil at 100 degrees. That's also why, when you have a kettle, for example, it doesn't actually boil water all the way to 100 degrees, right? It's actually a lot lower than that, because we don't drink pure water. Tap water is potable. A stage in the production of potable water is sterilizing. A gas is used to sterilize the water. So, chlorine gas plus water produces HCL and hydrogen chloride, and it's what it goes back as well. I kind of gave away the answer to part a, part a, part four, which was meant by the symbol here. It means it is a reversible reaction, which I hope that makes sense, because all we're trying to do here is purify or sterilize the water. If it, if this was not reversible, then how do we get the water back? We just get rid of the water. We sterilized it. For part five, I'll do that one as well. The reaction is at equilibrium and is exothermic. That means when this happens, heat is released, right? So, when the chlorine mixes with the water, heat is released. What happens to the equilibrium position when the temperature is increased? Now, if the forward reaction is exothermic, it means the reverse reaction has to be endothermic, which means heat is taken in. So, in other words, the forward reaction raises the temperature, the reverse reaction will reduce the temperature, so it requires energy. Another way to think of it is exothermic reactions release energy, endothermic reactions require energy, so they take in energy. If we increase the temperature, it means we're increasing the energy in the system. This means that the reverse reaction happens at a faster rate. So, let's say you need a certain amount of energy for each reaction of this. If we increase the energy, it means more of those reactions can take place every second. That means the equilibrium will shift to the left-hand side of the equation. Nice and easy. Describe a test to identify the gas used to sterilize the water. Well, if you go back, it was chlorine gas. Again, it's just a bit of knowledge, but it is the damp litmus paper, or if you have like litmus paper tests, or something along those lines, and essentially what happens is the paper will get bleached, which means it will go a stark white colour. Another stage in the production of potable water is filtering. Explain why potable water contains dissolved solids. So, the key thing about filtering is it's literally like running it through a grate, which means if it's dissolved in the liquid, it's not actually going to filter anything, and that's because the dissolved particles, dissolved solids, pass through the filter as they are very small. Are very small. That's pretty much it. An increase of greenhouse gases in the climate give one effect. Look at the news. Melting ice, rising sea levels, flooding, weather changes, loss of habitats, anything, right? The climate's changing, so anything to do with weather works for that. But now we have to explain how they cause global warming. So, first of all, greenhouse gases, they act like a blanket for the Earth. So, if I go back to my amazing diagram, the atmosphere is made up of a bunch of gases, and one really useful feature of these gases is they act like a blanket. So, as you might know, space is very, very cold. Shocking, I know. Now, if you notice, with a lot of the other planets in our solar system, if you look up the temperatures between night and day, they are massively different. So, in the daytime, they can go to like 100 degrees plus. In the nighttime, they can go to minus 100 degrees or worse. So, what that leads to, and the reason for that is the greenhouse gases, the lack of greenhouse gases in these planets. So, for example, Mars is a good example. It's quite close to us, relatively similar size, but it has massive temperature differences. The reason is, during the day, the sun is very, very hot, and it releases, well, light, right, and heat towards the Earth or towards a planet. Now, our greenhouse gases absorb a lot of this, a lot of the light, a lot of the heat, and that means that the air itself gets warm, but it doesn't mean that we get extremely, extremely hot like other planets in our solar system do, without the greenhouse gases. Then, when the sun goes away for nighttime, well, all of a sudden, this light is no longer being given, right? Now, for planets without a blanket, without greenhouse gases, that means all of the heat from the surface goes to space really, really quickly, because space is roughly negative 270 degrees Celsius. It's really bloody cold, which means it can cool down the planet really, really quickly if there's no greenhouse gases. But what we have, well, what do you, you know, what do you sleep under, right? You sleep under a duvet, right? Or you sleep with clothes on or something like that. That keeps you warm, because although it gets colder outside your bed, it's nice and warm inside your bed because of the blanket retaining a lot of your heat. So, the thing is, greenhouse gases causing global warming is a good thing. So, why is this a problem? It's a problem because what happens when you wear too many layers, right? What happens when your blanket is too thick at night in the summer, for example, and you, if you have two duvets on and you're wearing lots of clothes? You get really, really hot and sweaty. Now, of course, for you, that's just uncomfortable. What about for the entire planet? Well, when this global warming happens for the entire planet, it gets hotter and hotter and hotter. Well, then we have all of those problems we said before, where ice caps melt, sea levels rise, water evaporates, things like that. Places dry out. Places that used to have lots of water dry out, which means nothing can live there. Well, desert animals, I suppose, could. So, how is this caused? Well, I've explained it briefly, but here's the idea. Greenhouse gases allow short wavelength radiation to enter the atmosphere. The long wavelengths are absorbed by the greenhouse gases, which heats up the atmosphere. Now, these wavelengths are absorbed and re-emitted by the Earth at a longer wavelength. The reason for this is they lose energy. So, when the Earth absorbs them, it heats up the Earth, then the Earth will re-emit them. It will re-emit infrared radiation, but it's at a longer wavelength because some of that energy is lost. Remember, longer wavelength radiation has lower energy. So, the problem with this is that the greenhouse gases in the atmosphere, they don't allow it to pass back out because it's a longer wavelength. Remember, it absorbs long wavelengths. It lets short wavelengths through. So, now it's been re-emitted at a longer wavelength. It's no longer let out into space. Absorb this longer wavelength radiation, which increases the temperature of the atmosphere. And bear in mind, if the atmosphere gets hot, then the Earth gets hot as well because, again, it's like a blanket. If you have a hot blanket and you go underneath it, you'll get hot as well. This is hopefully a fairly nice question. It says, explain how planting trees reduces global warming. Now, remember, they've actually given you a hint. Planting trees reduces global warming. They've told you that because they're saying explain how. They can't ask you to explain how something impossible happens because it's not possible. So, let's think for a second. Imagine we knew nothing. The only thing we know from the question is that planting trees reduces global warming. Well, what have they told us in the last part of the question? Greenhouse gases causes global warming. So, what can we learn from that? Well, planting trees must reduce greenhouse gases. Okay, what gas do trees famously use? They use carbon dioxide. Carbon dioxide is a greenhouse gas. Do you see the logic, how we can build it up? We can use the question to answer the question. So, trees use carbon dioxide for photosynthesis. That gives me two marks. Why? Because I've said they use carbon dioxide and I've told them for what purpose. Carbon dioxide is a greenhouse gas that causes global warming. Hence, more trees means less greenhouse gases. There you go. So, I've used the question to answer the question. I know planting trees helps global warming. I also know from here greenhouse gases cause global warming. So, planting trees must reduce greenhouse gases. That's the logic, right? And that's what you can use to help answer these questions. A student investigates the rate of decomposition of hydrogen peroxide using three different catalysts, manganese dioxide, copper oxide, and zinc oxide. And this is the method. Yeah, wonderful, cool. The equation for the decomposition of hydrogen peroxide is hydrogen peroxide goes to water and oxygen. Describe a test to identify the gas. You're probably noticing a question, right? It's very similar to the one for hydrogen, but remember it's not a burning splint, it's a glowing splint, which is, of course, completely different. And what's the result? It will relight. So, it will catch on fire again, okay? Using 10 centimetres cubed of hydrogen peroxide solution gives less accurate results than using 20 centimetres cubed of the same concentration. So, basically, if we increase the amount of hydrogen peroxide, we get a more accurate reading. Why? Well, the reason is, what are we actually measuring here? We're measuring the amount of gas produced. So, in this case, the amount of oxygen produced, not the water. So, if we have more gas being produced, then this reading is going to be bigger. In general, in science, and especially if you want to do any sciences at A-level or at university, you want your measurements to be as big as humanly possible, because they give you a smaller error. There's a smaller percentage error. So, for example, this is all the way out to like 50 something. That's better than it being all the way down here somewhere. So, simply put, again, two things. It's a less accurate result because it's a smaller volume of gas produced, which means there's a larger error in reading. While we're talking about reading measurement, I should write, while we're talking about error, we might as well do the part three, which is suggest one possible source of systematic error. So, I mean, loads of different things. We are using a mass balance, for example, for this. So, it could be that there's a zero error there, for example. So, it's a wrong calibration in the balance. The timer itself could be too fast or too slow. So, again, a zero error in terms of the stopwatch, for example. The measuring vessel could also be incorrectly calibrated as well. I think the most common one is probably the mass balance being incorrectly calibrated. So, again, that's a zero error, which is a type of systematic error. For this, we have to plot the data from table three and draw a line of best fit. Let's go zero, zero, then we have 30. So, let's look at what this goes up in. It should go up in, so it's 50 here, and there's 10 squares. So, if we do 50 divided by 10, each small square is five seconds. We need to go to 30 seconds. We should also work out what this is. So, there's five squares between zero and 10, which means that must go up in twos. So, we're going to find 30 and 22. So, that'll be six squares, one, two, three, four, five, six. So, here, and we're going up to 22. So, that should be 11 squares right there. So, that's my second point, and keep going. 60 and 38. So, 60 should be just here. I already forgot it was 38, was it? Yep. So, that'd be one square before here. Then we have 90 and 41. So, that'd be two squares before this one. 41 would be halfway between those two points. Is that right? Okay, it's leveling off. Okay, 90 and 41. That is correct. Again, if you notice what I just did, if a value doesn't seem right, then definitely double check it just to make sure. 120 and 54. So, then that would be, oh, there's no easy one here. 105, 110, 115, 120, and it goes up to 54. That one there, because again, don't forget, goes up in twos. So, the vertical goes up in twos, the horizontal goes up in fives. Then we have 150 and 58, and last but not least, we have 180 and we have 60. So, that's 150, 55, 60, 65, 70, 75, 80, 60, right? I do worry about my memory. Okay, and we have to draw a line of best fit. Interesting. Now, drawing the line of best fit, again, it might be a bit tricky for me to do here, but what you'd want to do is you get a ruler. Mine is not going to be perfect, because this only goes up in fixed degrees, but you want to get your points and you want to try and make it so that an equal number of them are above and below the line. So, here, for example, there's way too many above the line. So, I might do something like that. That's a bit closer. Something like that. So, there's three below the line, three above, roughly. There's something along those lines. I just moved the entire thing. Okay, good. Something along those lines might be a good line of best fit. Although, it looks like a curve of best fit would have been better, but whatever. Yeah, something along those lines. Not bad, not great. Let's see. This is which cases gives the fastest rate of reaction? Give one reason. So, it's going to be the one that we just drew, because when they say rate, they're talking about gradient, about the steepness of the line. So, for this, that's really slow, faster, fastest. So, it's going to be the manganese dioxide, which is the one we just drew, because it has the steepest gradient. Remember, rate always refers to the gradient of the graph. The rate of reaction is not dependent on the volume of hydrogen peroxide solution. Why? Well, the reason is because the concentration is the same, right? Concentration is the same. There is the same number of particles per unit volume, which means the frequency of collisions is the same. Remember, if we're using the theory for reactions, which is a reaction happens when two particles collide with enough energy, then it's the frequency of collisions that increases the rate of reaction. Now, because the concentration is the same, that means it's the same number of particles per volume, which means the number of collisions they have with the catalyst is the same overall. We get a fat juicy five mark question to finish off on. So, it says determine the rate of reaction at 20 seconds, show you're working on figure four. So, what you need to do, and you get marks for this, is you need to draw a tangent at figure four. Now, there is a lot of leniency given here, because tangents are kind of subjective. This actually kind of looks right. Interesting. Again, don't forget, you do have quite a bit of leniency, so you don't have to be super, super accurate. Okay, I can't go back. That's nice. It's not bad. Move it slightly. There. Okay, I'm happy with that. And then you need to work out the gradient of this line. Now, I'm going to choose some points that are going through kind of corners. So, I'm going to choose that point there. What would be another good point? Oh, that point right at the end. And all we're going to do is we're going to do the change in height over the change in x. So, for this one, the y value is 5. For this one, the y value is unclear. So, it goes up in, so between 10 and 5, that's 5 units, and there's 10 squares. It goes from 0.5. So, that's 15.5, 16, 16.5. So, the change in y is 16.5 minus 5 over, now we need to do the change in the x coordinates. Let's use green for that. Well, that is, okay, so there's 5 squares between 0 and 10, so each square is 2. And here is 42. That's 42 minus 2. And let's have a look, shall we? 16.5 minus 5, 42 minus 2, to 3sf, so 0.288. And again, like I said, they give you quite a big range of values because the tangent thing is very, very subjective. So, what your examiner will do is they'll look to see if your tangent is reasonable. They will then read off the values and make sure you've done the gradient correctly. That brings us to the end, finally, of this paper. Although it's only an hour and 15 minute paper, I think the video has been, I'm curious to see how long the video is actually. But yeah, if you do want to see more of the chemistry paper too, then do let me know. If you want to see any of the other science papers, let me know. I'm also going to do the 2024 papers for maths. So, again, let me know if you want those more urgently than others. But I hope you have a wonderful day and I hope your revision is going well.