[Music] um uh hey everyone uh welcome to the uh to this xenons uh live academic session uh today we're gonna be doing uh physics and uh we have uh published author kareem above with us here today we're going to talk about gas laws and particularly today we're going to solve past paper questions and without further ado let's get started thank you very much again for the introduction pugs xenotypes it's nice to be back again yes today we are going to go through a few gas laws questions that are mixed in with some other topics as is normal for any examination paper so let's get stuck right in so question number four which comes from the may june 2016 paper for variant three in an industrial process a gas that is already at high pressure is trapped in a large cylinder by a piston figure 4.1 shows the gas the cylinder and the piston now guys that's a bit like this piston right here okay so if i was to seal one end of this piston with my thumb here is my gas and i could create a very high pressure by squeezing this in so the piston is pushed into the cylinder as the pressure of the gas changes its temperature remains constant now that is a humongous hint towards boyle's law p1v1 equals p2v2 if you remember from the lessons the idea in any experiment is not to change more than one variable at a time okay you do that because when we did the lesson we realized that volume uh temperature and the mass of gas all had an effect on pressure i mean look this is like a little toy bicycle pump if you've ever used a bicycle pump or a pump if you like if you've ever used a bicycle pump to pump up a football a basketball a netball the tire of your bicycle you move that in and out quickly anybody that's ever done that will know that they'll feel a lot of heat at the end therefore not only are you changing the volume but you're also changing the temperature here they're saying the temperature remains the same big hint towards change the volume see what happens to pressure so here we've got a graph figure 4.2 shows how the pressure of the trap gas depends on its volume now as i pointed out in the lesson before a lot of students think inverse proportion let me just draw on this looks something like oops my bad let me go back in inverse proportion looks something like so if that was my y-axis there's my x let me just go to the next page well i'll do it on the next page they think it's a straight line like that no it's not all right inverse proportion is the shape that you are seeing in front of you right now when the volume goes up the pressure goes down and then it says describe the relationship shown by the graph between the pressure of the gas and the volume it occupies explain this relationship in terms of the molecules and that's been very specific so not only are they asking you to describe boyle's law they're asking you to explain how that relates to the molecules themselves so if you imagine like this piston that i've got here if i stick my thumb at the end and i try to push this all the way in if you look at it it pops straight back out and the reason for that is gases are compressible i'm trying to compress it but when i let it go the gas is pushing it straight back out in other words this piston can move backwards and forwards all right so what they're wanting here is for you to understand that p1 v1 equals p2 v2 in other words p times v is a constant you can write p1 v1 equals p2 v2 tele examiner look when the volume gets bigger the molecules are more spread out and there are less frequent collisions all right so if you look at this as the volume gets bigger the pressure's going down as the volume gets bigger the molecules spread out there are less frequent collisions and therefore you've got a smaller pressure if you want to flip that you could say look as the volume decreases pressure goes up because there's less space and there's more frequent collisions you're welcome to flip it either way it's not a big deal okay so i hope that makes sense then it says determine the volume occupied by the gas when it's released into the atmosphere at a pressure of 0.10 megapascals you'll need data from 4.2 well this is where we need to come back here and what the examiner's after is that you understand p times v is a constant so let me try to pick a number that looks kind of nice and easy to look at let me find one i'm zooming in here yep okay i'm going to come down here and down here 0.20 meters cubed if i was to go all the way along all right that to me is spot on one it's not well drawn there it's one mega pascals so if i go across from zero point now you could do this with any point by the way i'm just looking at one that gives me a nice number that isn't in between anywhere and i need to use half boxes i'm just looking okay 0.2 what do you give me you give me one megapascal so what do i need to do with that well by using a calculator you would do 1 times 0.2 which of course is 0.2 so what you've got guys is you've got let me just do this p1 v1 equals p2 v2 which effectively gives you 1 times 0.2 equals pressure sorry volume pressure 2 which is 0.1 which is given us times volume 2 and our job here is to find volume 2 and the way we would do that is we would do 1 times 0.2 i'll just do it here 1 times 0.2 divided by 0.1 and guys what you'll get is you'll get 2 meters cubed or that's what i think we should get yep so don't worry what the math scheme says we're basically getting two oh well it's actually two meters cubed and depending on whether you actually put in the mega pascals or not you could have ended up having two times ten to the five but it is two meters cubed i hope that makes sense and it comes from p1 v1 equals p2 v2 let me just get rid of this okay next an unused cylinder of the compressed gas is moved into a cold warehouse the temperature of the gas decreases state how the decrease in the temperature affects the motion of the gas molecules again hopefully from the lesson we did previously you'll understand well look kinetic energy is related to temperature so the smaller the temperature the less kinetic energy the molecules have in other words they slow down what about the pressure of the gas in the cylinder well look if they slow down they have less kinetic energy they're colliding less often which means the pressure is going to go down we okay with this pugs if you have any questions you want me to crack on brilliant okay so next question four a from october november 2016 paper four part one in an experiment cold water is pulled poured into a bowl made of an insulating material the container is placed in a draft free room in other words there's no wind or air conditioning windows are closed after several hours the volume and the temperature of the water are found to have decreased naaman describe the process that causes the decrease in volume of water and explain why the temperature of the water decreases now guys this is a topic that i've not covered with you but it is now going to mix in with gas laws we are of course talking about evaporation look a lot of students think evaporation happens only when water is boiling if you're like me and you sleep with a glass of water better still a bottle of water by your bedside if you wake up in the morning you will see condensation at the top of the bottle you do not have underfloor heating if you do that's pretty cool but most people don't so why is there condensation at the top of the bottle it's because the water is trying to evaporate water will evaporate at any temperature don't believe me after this lesson going up in the freezer and you see cold smoke as my kids would call it come out into your face it's evaporated water even from a freezer all right so yes when water is boiling it's more noticeable but even from a freezer water will evaporate and if you think about guys um you've had a shower and you come out the shower and you'll see steam in the walls trust me you were not having a shower in boiling water all right because you'd be going straight to the hospital before you'd even finish that shower so the point is evaporation occurs at any temperature it's more noticeable when things are boiling so what happens in evaporation the most energetic molecules have overcome the forces of attraction and they've left the surface of the liquid so what does that mean well think about it you've got three states of matter solid liquid gas a liquid is turned into a gas gases have more energy so if gas has more energy what's left behind a liquid with less energy guys this is why when we play sports and we sweat it cools us down if it's a hot summer's day and you're playing football or wherever you're playing sport and you throw water over yourself it's to cool you down but one of the main reasons it cools you down is not what you think yes if you put very cold water over yourself yes it was cold to begin with but what happens is heat from your body will evaporate that water now think about what happens a liquid is just turned into a gas where do they get the energy from your body your body has given energy to the water it's evaporated so temporarily at least you're feeling cooler why do i say temporarily if you're going to keep running around playing football you're going to heat up or maybe as a kid maybe this still happens and you're embarrassed to admit it but your mother you've got a fever on your mother takes a cold towel and pours over it puts over your forehead why heat from your forehead when you've got this high temperature will evaporate the water on the towel it will escape the liquid is turned into a gas more energy is gone and leaving less energy behind so that's exactly what i've explained there that's exactly what that answer says evaporation highest energy molecules have escaped leaving less energetic molecule behind so the overall temperature of the remaining liquid decreases i hope that makes sense in a second experiment using the same apparatus and the same initial amount of cold water as in a an electric fan blows air over the top of the bowl so now you've got a draft predict and explain how the results of this experiment compare with the results of experiment a now if you've ever had a coffee starbucks costa whatever okay free shout out to starbucks and costa pugs i hope they're paying you for this show okay um if you've ever been to a coffee shop or hot chocolate you know i love hot chocolate and there's a lid on it what do you do if you really want to drink it quick you take the lid off and you blow over it no there is no air conditioning coming out your mouth okay that doesn't happen but what you're doing is any molecules have evaporated you're blowing them away so they cannot fall back inside look guys if you've ever been bored okay and okay i've got a glass of water here but let's just imagine that was a glass of a fizzy drink okay i could give shout outs to fizzy drink but it pays enough but let's say it was a fizzy drink if you ever look at the top of a fizzy drink you'll have you'll see can a gas molecules bounce up and then bounce back down okay if you've ever been that bored you'll see this happening now that's visible to us you can't see water molecules escape all right but if you've ever had a lid on top of a coffee and you take lid off you'll feel condensation on the inside that was trying to escape so the point is the lid is there for that reason if you try to blow that away now as it's popping up what's happening is you're blowing the evaporated molecules away therefore they cannot fall back inside and the whole thing becomes cooler if you've ever been in humid weather you'll know that's well in my opinion in my humble opinion that's the most horrible weather there is because you feel sweaty and horrible all right because any moisture near as your body's trying to cool down it just sticks to you and it gives you that horrible feeling unless of course you're on holiday and you don't care okay so what happens in this experiment greater decrease in temperature in a why because the fan removes the vapor it blows it away it reduces the humidity just like i've said therefore there's more evaporation if there's more evaporation it's going to cool down a lot more bugs any questions here or are we good to keep going what's going on cool so next one a third experiment the same initial amount called water now look look at all these experiments they're treating it like we did gas laws they're only changing one thing at a time so same initial amount of cold water okay poured into a metal ball this time the metal ball is the same size and shape as a ball using a see that they keep keeping everything the same but this time they've only changed the fact that it's metal one variable at a time guys okay that's how we do experiments compared with experiment a what can you tell me about the decrease in temperature okay the decrease in temperature sorry is less than the same time explain why well guys this time metal is a very good conductor so what's going to happen here is heat is going to pass from the liquid or from the surroundings raising the temperature all right so like if the room is hotter that heat is going to pass into the liquid raising its temperature that's why the decrease in temperature is less so let me give you an analogy that i give students quite a lot um let's say i asked you to dig a hole right you go to the beach nice sunny day i asked you to dig a hole one meter deep one meter wide uh one meter long now i want you to imagine that to be like evaporation so you're digging that hole but unbeknown to you i'm sweeping some of it not all of it back inside well if i'm sweeping some of it back inside what's happening here there is in the analogy the temperature's going back up so whilst the evaporation is trying to cool it down the temperature is actually increasing again because heat is being gained from the surroundings from the metal container because it's a good conductor of thermal energy all right the only time that the water would lose energy to the room if the water was warmer than the than the room to begin with but if the water's cold it's going to gain heat it's a common misconception you know when i teach students uh you talk about latent heating stuff which is coming i think you know we've done that actually and students will talk about ice creams and they'll go why does it melt or you know why do we need to make sure the ice cream is insulated or something cold is insulated and they'll say to stop heat being lost to the surroundings and i'll say when's the last time you stood and put your hand above an ice cream to go ah that's so warm guys cold things don't lose heat to the surroundings cold things gain heat from the surroundings that's why the metal conductor here is decreasing how quickly it cools down because it's gaining energy i hope that makes sense yeah next here's a boyle's law question a common boyle's law question question 5a from february march 2016 paper 4 variant 2. a student carries out an experiment to find the relationship between the pressure and the volume of a fixed mass of gas the table contains four of our sets of measurements use the data in the table to suggest how the relationship between the pressure and the volume in this experiment explain how you reach your conclusion guys a lot of students look at this and go well i don't know what's happening uh pressure goes up volume goes down that's not what they're after remember p times v is a constant so what they want you to do is multiply 250 by 30. 500 times 15.2 750 times 9.8 a thousand times 7.6 and what you'll get is all the values are roughly speaking 7500 in other words p times v is a constant now you might say why did i just say roughly speaking because it's an experiment you get errors things go up a bit things go down a bit but broadly speaking if you were to plot the graph it would be that curved graph okay the inverse proportion graph not the straight line but high pressure low volume low pressure bigger volume okay can we revisit that for a second let's just show people how it looks absolutely absolutely so there's the boyle's law graph right here so what a lot of students think is the graph looks something like well that's terrible give me let me do that again um what a lot of students think guys is it's a straight line i really should try to get a ruler for computers i wonder if there is one um but let me just erase the top line but in actual fact it's a curved line you okay with that pugs yeah okay so what they are talking about here in that table is exactly this okay and when you multiply them together just like we did in that first exam question we multiplied two numbers together and then divided it by the new i think it was a pressure in that question to get a new volume p times v is a constant that's the whole point about boyle's law so if the pressure doubled the volume halves or if the volume became a quarter the pressure would be a quart you know four times as much all right when you multiply them together you're always going to get the same answer okay or vice versa i hope you understand what that means i don't know if those chocolates still exist okay where if you've got white chocolate on the outside it was dark on the inside and if you remember it said it was lightened a white chocolate inside vice versa okay p times v is a constant okay these are just all different ways of saying the exact same thing state the property of the gas apart from the mass that remains constant during the experiment you know the exam has been a bit naughty there right because a lot of students would say mass especially when i've taught them over a period of time ago look guys whatever's in that pump we don't increase the gas we just leave it the same but what will remain the same in boils or remember we did earlier temperature we're only ever going to change one variable at a time and then part b slightly not fully related a lake is five meters deep the density is a thousand calculate the pressure at the bottom pressure equals density times gravity times height so this is the pressure formula for a fluid you've got the depth you should know gravity is 10 and you know the the density of water is a thousand plug in the numbers and you get 50 000 pascals or 50 kilopascals okay and then a bubble of gas escapes from the mud at the bottom of the lake and rises to the surface please take one row in the table to indicate what happens to the volume the mass and the density of the bubble in the gas so look they're actually mixing density and you know knowledge about gas laws as well so you know if this bubble is going to go to the top you know i don't know if you've ever tried talking underwater if the bubble when it comes to the top it gets bigger in fact that could be really crude here and let's say you've lit off some methane gas in the bathtub not the best example uh probably a smelly example or in a swimming pool when that bubble gets to the top it expands it gets bigger so what's going to happen to the volume of the bubble for sure the volume is going to increase all right what's going to happen to the mass of the bubble we'll look the gas that comes out the mass is going to stay the same what's going to happen to the density well this is slightly different from gas laws but when volume increases density decreases okay so if i was to tick the boxes now the volume of the bubble goes up the mass of the gas stays the same because it's stuck in that bubble the density goes down think of a hot air balloon guys why does a hot air balloon rise because it's less dense than the air around about it and when you heat it up the balloon becomes bigger so it becomes even less dense and when things are less dense they will always rise look at the topics of conduction convection and radiation and also look up dense equals mass over volume so what you'll have saw there guys is gas laws but with a mixture of some other topics and that just happens that's the nature of exams you're always going to get that and if i'm not yeah if i'm not mistaken that is all the questions have any questions to ask or or thank you everyone for tuning in and uh catch us again on next tuesday uh for updates uh you can subscribe to the youtube channel make sure you press the bell icon uh follow us on our instagram page and to ask us questions join these uh student community on discord thank you see you all there thank you very much