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Current time:0:00Total duration:12:36

Video transcript

all of the phase changes we've been doing so far have been under constant pressure conditions and in particular with the problems that I've been doing with water phase changes in the last couple of videos it was that it was at atmospheric pressure at least at sea level atmospheric pressure or at one atmosphere so it was done will I'll explain this diagram in a second but we all know that in the universe pressure isn't always constant and it definitely isn't always constant at one atmosphere one atmosphere was defined is the pressure at sea level on earth obviously pressure will vary wildly if you go to smaller planets or larger planets or atmospheres would have thicker atmospheres or if we're just doing different types of applications dealing with gases and liquids and solids so what I've drawn here is a phase diagram let me write that down phase diagram phase diagram and there are many forms of phase diagrams these are this is the most common form that you might see in your chemistry class or on some standardized test and what it captures is is the different states of matter and when they transition according to temperature and pressure and this is a phase diagram for water so just to understand what's going on here is that on this axis I have pressure pressure on the x axis I have temperature and at any given point this diagram will tell you whether you're dealing with a solid so solid will be here solid a liquid will be here liquid or gas or gas for example if I told you that I was at oh I don't know if I was at 0 degrees let's say D 0 degrees is right there if I'm at 0 degrees Celsius and 1 atmosphere where am I so 0 degrees 1 atmosphere am i right at that point right there so I'm at a boundary point between solids and liquids at 1 atmosphere of pressure right this is when we're at one atmosphere of pressure so this coincides with our traditional notion of when ice freezes or when it melts at 0 degrees if we made the pressure higher if you made the pressure higher what happens well then ice starts melting at a lower temperature right so this is pressure going up so pressure going up would say I don't know what this is this is maybe 10 atmospheres 10 times its atmospheric pressure at sea level then all of a sudden the temperature at which solid turns into liquid right this transition is solid to liquid the temperature would just that that happens will go down likewise if we lower the pressure if we go to Denver you know it's a mile high pressure is lower because we have less of the atmosphere above us then all of a sudden the boiling point I'm sorry the freezing point increases so the freezing point will be something above one degree and this isn't drawn complete to the scale but the idea is your ice would actually freeze a little bit faster and would freeze at a higher temperature in Denver than it would is at the at the bottom of the Dead Sea or the at the Death Valley it's some very low below sea level point on the planet now this transition is the transition between well between anything and gas and we're very familiar this is one atmosphere and remember this is water we're dealing with this is the diagram for water so at one atmosphere this is kind of the stuff that we're used to seeing let me draw a line here so at 1 atmosphere zero degrees zero degrees is where is where solid or ice turns into liquid water and then we go up here so we keep going the higher higher higher temperature and then here this would be since we're at one atmosphere this is 100 degrees 100 degrees Celsius right there and that's the point it at one atmosphere pressure where liquid turns into gas or water vaporizes or the liquid boils all of those are acceptable ways to think about that but what happens when we go to low pressure once again let's take our little trip to Denver right so that's Denver right there Denver it's not that drastic I'm just doing that for education purposes or even better let's say Mount Everest Mount Everest Mount Everest very low pressure there then our freezing point we already said that goes up when you lower the pressure and your boiling point goes down so it's much easier to boil to boil something on the top of Mount Everest than it is to boil it at the bottom or at the lowest point in Death Valley or the Dead Sea and the reason the way the intuition behind that is if I have a if I have a liquid if I have a liquid a bunch of molecules in liquid form and they're touching each other but they have enough kinetic energy to move past each other so they're moving there they're flowing past each other they're kind of rubbing up against each other one of the reasons why they don't just evaporate why you know this guy doesn't just jump up there is that there's air above him there's air pressure and air pressure we've learned about this when we did PV NRT that's a bunch of gas molecules and the pressure they're creating is essentially caused by their temperature and their kinetic energy and they they sit there and they bounce and they they essentially keep these heavier molecules from going up they keep them from from from essentially separating from each other and turning into a gas so the more pressure you have the harder it is for these guys to escape on the other hand if you have very very if we're a vacuum if we're doing this on the surface of the moon and there's none of these guys there then just a little slight bump even though this guy is still a little bit attracted to over here they're still attracted to each other but just a little bit of bump since there's no pressure up here for on the floor for some moon might allow this guy to to escape and and and go straight to a gas so when you lower the pressure it's just that much easier to go from liquid to gas or even from solid to gas and you might say Sal that's a bizarre concept solid to guess and it turns out if you get to low enough pressures here I mean let's say this is this is actually there's probably not stuff here this is probably close to a vacuum right here you could go from I so if you took ice and you were on the moon and you are at the right temperature this is you know maybe some negative negative degree Celsius temperature I don't know what the exact temperature is your ice on the moon would go directly from ice to a gas because there's no there's this huge vacuum here so these molecules will say hey so all the space to fill and if they just get bumped a little bit they're just going to escape and turn into a gas and you might say Oh Sal that's that's a strange phenomenon it only exists on the moon and because to to rebut that comment I've drawn the phase diagram for carbon dioxide carbon dioxide what we are you it's all around you you're exhaling it as we speak your plants in the room or hopefully inhaling it but carbon dioxide at one atmosphere has a very different behavior than water this is carbon dioxide at one atom so and just so you know the this scale is definitely not drawn to scale the difference between one atmosphere and five atmospheres is not the same between five atmospheres and seventy-three likewise this is not drawn to scale here this is a much larger distance than this so if I had to really draw it to scale out to stretch this chart out or do a logarithmic chart or something but anyway I was talking about carbon dioxide so this is carbon dioxide solid and this is gas and this is liquid carbon dioxide so at one atmosphere so if it didn't like you know let's say you live at sea level I looked your in your New Orleans I guess it's a little bit below sea level that's where I grew up if you were able to get your fridge down to minus eighty degrees Celsius the carbon dioxide would actually freeze and you're actually not too unfamiliar with that all right at least you haven't been if you've gone to some I don't know if they still use it to make for smoke machines or for visual effects on stage but this is dry ice dry ice it's frozen carbon dioxide and then as soon as you get above if you're at if you're at a kind of a sea level atmospheric pressure as soon as you get above this minus seventy eight and a half degrees Celsius it sublimates to gas so that process where you go straight from a solid to a gas is sublimation sublimation and that's why I dry ice when you see it there's you don't see liquid dry ice or I mean you don't see it at standard pressures I've never seen liquid carbon dioxide in or in fact order to get liquid carbon dioxide you have to get above five atmosphere so you have to get above five times the sea level pressure on earth and you're really not going to see that in natural conditions on earth you might see that on Saturn where you have tremendous pressures because of the gravity and all of the atmosphere above you so this is a kind of thing liquid carbon dioxide you might see I don't know if Jupiter actually has carbon but you'll probably see it on other huge massive planets that are gas giants but on earth this process is just called sublimation it's just a neat word or it's sublimating it's going straight from solid to gas and it's something you've seen with dry ice now there's a couple of other interesting points here and you've probably already noticing them and this right here is called the triple point because right here at this well in the case of carbon dioxide at five atmospheres and minus 56 degrees Celsius the carbon dioxide is in a state of equilibrium between the ice the liquid and the gas it's a little bit of all of the three and if you just nudge it in one direction or another by nudging the pressure the temperature it'll go in that direction similarly waters triple point triple point is right here it's at a much lower pressure than we're used to dealing with this is 611 or 0.6 11 kilo Pascal's or just 611 Pascal's which is you know not five thousandths of an atmosphere so if you go down to five thousandths of an atmosphere and you go a little bit above a little bit above zero degrees Celsius you're at the triple point of water where water can take on any of these states where it's if you just nudge in one direction or another now the other interesting point on these charts is up here this is the critical point sounds very important critical point and that's the point at which if you increase the temperature beyond that or the pressure beyond that you're dealing with a supercritical fluid supercritical fluid sounds very exciting so above here you have a supercritical fluid so very high temperature very high pressure so it's so high temperature that it wants to be a gas but you're putting so much pressure on it that it wants to be a fluid so it's a little bit of both and actually in the case of water supercritical water is actually used as a solvent because you can imagine it's it's kind of like liquid water and that things can dissolve in it but it's so high temperature and it can it can diffuse into solids that it's really good at just you know getting whatever you want out of whatever you're trying to clean or somehow so get into or I guess get get salt you know put into the water so this is supercritical fluid and so it's a fun thing to think about but anyway I just wanted to expose you to these two these phase diagrams actually let me just let me just everything I've done so far was that a constant pressure and I change the temperature but you can also read them the other way if I'm at a hundred degrees and I go from let's say I'm at 110 degrees or we're at sea level is comfortably in the gaseous phase for so this is 110 degrees for for water its water vapor but if I were to increase the pressure and keep increasing the pressure maybe I you know dig a hole or something or going to the into the ocean then it's going to it's going to condense into water or it's going to condense into a liquid if I did that experiment here well that's probably let's say I did the experiment I did experiment here when I increase the pressure I'm going to reverse sublimate and I think I wrote down a word for what that is let me see if I wrote it down someplace oh no I didn't I didn't write it down but essentially it's it's something like condensed but my brain did though the word is is escaping me at the second it's something on the word of condensing or falling together anyway I forget the word but it'll go straight from a gas to a solid so these are pretty neat diagrams actually tell a lot about different substances and then tell you what happens when the pressure or the temperature changes