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Current time:0:00Total duration:19:24

Video transcript

I think we're all reasonably familiar with the three states of matter in our everyday world they're at very high temperatures you get a fourth but the three ones that we normally deal with ours are things could be a solid a liquid I'll do that in blue well it doesn't have to be blue or it could be a gas and we have this general notion and I think water is the example that always comes - at least in my mind is that solid happens when things are colder relatively colder and then as you warm up you go into a liquid state and as you warm up even more you go into a gaseous state so you go from colder to hotter and in the case of water when you're a solid you are you're when you're solid you're ice when you're a liquid you're you could I mean you know some people would call ice water but let's call it liquid water liquid water I think we know what that is liquid water and then when it's in the gas state you are essentially your vapor or steam vapor vapor or steam so let's think a little bit about what what at least in the case of water and the analogy will extend to other types of molecules but what is it about water that makes it solid and when it's colder what allows it to be liquid and I'll be frank some liquids are kind of fascinating because you know you can ever kind of you can never nail them down this is the best way to view or or gas so let's just draw a water molecule water molecule so you have oxygen there you have some bonds to hydrogen and then you have two extra pairs of valence electrons in the oxygen and we did a couple of videos ago we said hey oxygen is a lot more electronegative than the than the hydrogen that it likes to hog the electrons so even though that you know this shows that they're they're sharing electrons here and here right at both sides of those lines you can kind of view that you know hydrogen is contributing an electron and oxygen is contributing electron on both sides of that line but we know because of the electronegativity or the relative neck electronegativity of oxygen that it's hogging these electrons and so the electron spent a lot more time around the oxygen they do around the hydrogen and what that results is is that on the oxygen side of the molecule you end up with a partial negative charge and we talked about that a little bit on the hydrogen sides of the molecules you end up with a slightly positive charge you end up with a slightly positive charge all right they're slightly positive now if you have very little I guess the best thing is if these molecules have very little kinetic energy they're not moving around a whole lot then what these these the the positive sides of the hydrogen's are very attracted to the negative sides of oxygen in other molecules so let me draw some more molecules so when we talk about the whole state of the whole matter we actually think about how the molecules are interacting with each other not just how the atoms are interacting with each other within a molecule so let's say that I just drew one oxygen let me copy and paste that whole face but I could do multiple oxygens and let's say that that hydrogen is going to have a is going to want to be near this oxygen because this has partial negative charge this has a partial positive charge and then I could do another one right there and then maybe we'll have and just to kind of make the point clear you know you have two hydrogen's here maybe an oxygen wants to hang out there I'll do it in the so maybe you have an oxygen that wants to be here because it's got its it's got its partial negative here and it's connected to two hydrogen's right there that have their partial positive but you can kind of see a lattice structure being you know let me draw the let me draw this these bonds that these polar bonds that start forming between the particles and these bonds they're called polar bonds because the molecules themselves are polar and you can see it forms this lattice structure and if the if the each of these molecules don't have a lot of kinetic energy or if you say the average kinetic energy of this of this of this matter is fairly low and what do we know is average kinetic energy well that's temperature then this lab structure will be solid so these molecules will not move relative to each other they're all you know I could draw a gazillion more but I think you get the point that we're forming this kind of this this this fixed structure and while we're in the solid state as we add kinetic energy as we add heat what it does to the molecules is it just makes them vibrate around a little bit so the kinetic energy just makes these you know if I was a cartoonist out the way you draw like a vibration is put a little quotation mark there that's not very scientific but they would vibrate around they would they would buzz around a little bit growing arrows to show that they're vibrating doesn't have to be just left right it could be up-down but as you add more and more heat in a solid these molecules are going to keep their structure so they're not going to move around relative to each other but they will they will convert that heat when heat is just a form of energy into kinetic energy which is expressed in these by as the vibration of these molecules now if you kind of if you make these molecules start to vibrate enough and if you put enough kinetic energy into these molecules what do you think is going to happen well this guy is vibrating pretty hard and he's vibrating harder and harder as you add more and more heat this guy is doing the same thing at some point these polar bonds that they have to each other are going to start not being strong enough to contain the vibrations right and once that happens once that happens the molecules let me draw a couple more once that happens the molecules are going to start moving past each other right they'll start moving past each other so now all of a sudden the molecule will start shifting the molecule will start shifting but they're still attracted to maybe this side is moving here that's moving there you have other molecules moving around that way but they are still attracted to each other right even though the we've gotten to the kinetic energy to the point that the vibrations can kind of break the bonds can break these these the bonds between the polar sides of the molecules it still our vibration or kinetic energy on for each molecule still isn't strong enough to completely separate them they're starting to slide past each other and this is a hat essentially what happens when you're in a liquid state is that you have a lot of atoms that are just they want to be touching each other but they're sliding they have enough kinetic energy to slide paths each other and break that solid that solid lattice structure here and then if you add even more kinetic energy even more heat to the to the to the at this point it's a solution now you're going to break they're not even going to be able to stay together they're going to be able to stay near each other if you add enough kinetic energy they're going to start looking like this they're going to completely separate and then kind of bounce around independently especially independently if they're an ideal gas but but in general a gas is they're no longer touching each other they'd a you know maybe they might bump into each other but they have so much kinetic energy on their own that they're all doing their own thing and they're not touching and I think that makes kind of intuitive sense if you just think about how it what a gas you know you for example it's hard to see a gas why is it hard to see a gas because the molecules are much further apart so they're not they're not they're not acting on the light as in the way that a that a liquid or a solid wood and if we keep thinking making that extended further a solid well I probably shouldn't use the example with with ice because ice or water is one of the few situations where the solid is less dense than the liquid that's why ice floats and that's why icebergs don't just all fall to the bottom of the ocean and and ponds don't completely freeze solid but you can imagine that because a liquid is in most cases southern water less dense that's another reason why you can see through it a little bit better or it's not diffracting well I won't go into that too much then then maybe even a solid but the gas is the most obvious is that and it is true with water the liquid form is definitely more dense than the gas form that the gas form will kind of that the molecules are just going to jump around not touch each other and because of that yet more light can kind of get through the substance now the question is how do we measure the amount of heat that it takes the amount of heat that it takes to do this to water and to explain that I'll actually draw of a phase change diagram which is a fancy way of describing something fairly fairly straightforward so let me say that this is the amount of heat I'm adding and this is the temperature and then we'll talk about the states of matter in a second so heat is often denoted by Q sometimes people will talk it will you know change in heat they'll use H lowercase uppercase H they'll put a delta in front of the H Delta just means change in and sometimes you'll hear the word enthalpy and let me write that because it was a word that I used to you know I'd say what is enthalpy they it sounds like they're using it enthalpy it sounds like empathy but it's it's quite a different concept at least as far as my neural connections can make it but enthalpy is kind of it is closely related to heat its heat content its Heat whoops its heat content heat content for our purposes when you hear someone say change in enthalpy you should really just be thinking of change in heat I think this word was really just introduced to to confuse chemistry students and introduced a non-intuitive word into their vocabulary the best way to think about it is heat content change in enthalpy is really just change in heat and just to remember you know all of these things whether we're talking about heat you know kinetic energy potential energy enthalpy they you know you'll hear them in different contacts and you're like oh I thought I should be using heat and they're talking about enthalpy or these are all forms of energy and these are all measured in joules and they might be measured in other ways but the the traditional ways joules and energy is the ability to do work and if you what's the what's the unit for work well it's joules force times distance but anyway that's a side note but it's good to know this word enthalpy especially in a chemistry context because it's used all the time and it can be very confusing and not intuitive because you're like where did this info what you know I don't know what enthalpy isn't in my everyday life let's just think of it as heat content because that's that's really what it is but anyway on this axis I have temperature or not I have heat right so this is when I have very little heat and I'm increasing my heat and this say is temperature temperature temperature now let's say at low temperatures I'm here and as I as I add heat my temperature will go up right temperature is average kinetic energy let's say I'm in the solid state here and I'll do the solid state in purple and so as I add heat I already was using purple I'll use I'll use magenta so as I add heat my temperature will go up right heat is a form of energy and when I add it to these molecules as I did in this example what did it do it made them vibrate more or it made them have higher kinetic energy or higher average kinetic energy and that's what temperature is a measure of average kinetic energy so as I add heat in the solid phase my average kinetic energy will go up and let me write this down this is in the solid phase solid or the solid state of matter now something very interesting happens and let's say this is water let's say this water so what happens at at this is zero degrees I'll do zero degrees or which is also 270 3.15 Kelvin zero degree Celsius but let's say that's that line what happens to a solid well it turns into a liquid ice melts and they're not all solids this we're talking in particular about water about h2o so if we're so this is ice in our example all solids aren't ice although you know you could you could think of you could think of a rock as solid magma because that's what it is right you can think of I mean what we could I could take that analogy a bunch of different ways but what an interesting thing that happens is at a hundred degree or at a zero degrees right the-the-the depending on what direction you're going either the freezing point of water or the melting point of ice something interesting happens as I add more heat I the temperature does not go up as I add more heat the temperature does not go up for a little period let me draw that for a little period the temperature stays constant so and then and then while the temperature is constant stays a solid we're still a solid and then we finally turn into a liquid then we finally turn into a liquid let's say right there so we added a certain amount of heat and just data solid but it got us to the point that the ice turned into a liquid it was kind of melting the entire time that's the best way to think about it and then at once we keep adding more and more heat than the liquid warms up to so then the liquid the liquid warms up now we get to what temperature becomes interesting again for water well obviously 100 100 degrees Celsius or 373 let me draw it like this or 373 degrees Kelvin I'll do it in Celsius because that's what we're familiar with what happens that's that's the heat that's the temperature at which water will vaporize or which water will boil but something happens and you know why isn't you know that's they're really getting energy kinetically active but I just like when you went from solid to liquid there's a certain amount of energy that you have to contribute to the system and actually it's a good it's a good amount at this point where the water is turning into vapor but it's not getting any hotter so it's turning into vapor but it's not getting any hotter so yeah we have to keep adding heat but notice the temperature didn't go up and we're gonna talk about in a second what was happening then and then finally after that point where we're completely vaporized or we're completely steamed then we can start getting hot the steam can then get hotter as we add more and more heat to the system so the interesting questions I think it's intuitive that as you add heat as you add heat here we're going to get our temperature is going to go up but the interesting thing is what was going on here we were adding heat so over here returning our heat into kinetic energy temperatures average kinetic energy but over here what was our Heat doing well our heat was was not adding kinetic energy to the system it was the temperature was not increasing but the ice was going from ice to water so what was happening at that state is that the kinetic energy the heat was being used to essentially break these bonds to break these bonds and essentially bring the molecules into a higher energy state so you know you're saying Sal what does that mean higher energy state well if there wasn't all of this heat and all of this kinetic energy these molecules want to be very close to each other right for example I want to be close to the surface of the earth when you put me in a plane you have put me in a higher energy state I have a lot more potential energy I have the potential to fall towards the earth likewise when you move these molecules apart and you go from a solid to a liquid they want to fall towards each other but because they have so much kinetic energy they never quite are able to do it but their energy goes up their relative their potential energy is higher because they want to fall towards each other by falling towards each other in theory they could do some work so what's happening here is is when where when when we're contributing Heat and this actually this amount of heat we're contributing it it's you know we could either call it when you're going in it's called the heat of fusion heat of fusion because it's the same amount of heat regardless of how much direction we go in when we go from solid to liquid it's kind of we could view it as the heat of melting right it takes it's the heat that you need to put in to melt it the ice into liquid when you're going in this direction it's the heat you have to take out of the zero degree water to turn it into ice so it's the heat that you're so you're taking that potential energy and you're bringing the molecules closer and closer to each other so here so the way to think about it is right here this heat is being converted to kinetic energy then one where when we're at this phase change from solid to liquid the heat is being used to add potential energy into the system to pull the mark the to pull the molecules apart to give them more potential energy if you pull me apart from the earth you're giving me potential energy because I gravity wants to pull me back to the earth and I could do work when I'm falling back to the earth I could I don't know you know a waterfall does work it can move a turbine I could you could have a bunch of falling Sal's move vine as well and then once you are fully illiquid then you just become a Worman Worman liquid now now the heat is once again being used for kinetic energy right you're making the water molecules move past each other faster and faster and faster to some point where they want to they want to just completely disassociate from each other they want to not even slide past each other just completely jump away from each other and that's right here this is the heat of vaporization so this is the heat of vaporization heat of vaporization and that the same idea is happening before we were sliding next to each other now we're pulling them apart all together so we're so they could they could definitely fall closer together and then once we've added this much heat now we are we are where we're just heating up the steam or just heating up the gaseous water and it's just getting hotter and hotter and hotter but the interesting thing there and I mean at least the interesting thing to me when I first learned is that there there are these states that you can have whenever I think of zero degrees water I oh say oh it must be ice but that's not necessarily the case if you start with water and you make it colder and colder and colder to zero degrees you're essentially taking heat out of the water you can have zero degree water and it hasn't turned into ice yet and likewise you could have 100 degree water that hasn't turned into steam yet you have to add more energy you can also have 100 degree steam you can also have a zero degree water anyway hopefully that gives you a little bit of intuition of what the different states of matter are and then the next problem we'll talk about how much heat exactly it does take to to move along this line and maybe we can solve some problems on you know how much ice we might need to make our drink cool