1 00:00:00,000 --> 00:00:00,640 2 00:00:00,640 --> 00:00:04,149 We know that when we have some substance in a liquid state, 3 00:00:04,150 --> 00:00:08,789 it has enough kinetic energy for the molecules to move past 4 00:00:08,789 --> 00:00:11,529 each other, but still not enough energy for the 5 00:00:11,529 --> 00:00:15,099 molecules to completely move away from each other. 6 00:00:15,099 --> 00:00:16,530 So, for example, this is a liquid. 7 00:00:16,530 --> 00:00:18,960 Maybe they're moving in that direction. 8 00:00:18,960 --> 00:00:21,589 These guys are moving a little bit slower in that direction 9 00:00:21,589 --> 00:00:24,489 so there's a bit of this flow going on, but still there are 10 00:00:24,489 --> 00:00:25,539 bonds between them. 11 00:00:25,539 --> 00:00:28,059 They kind of switch between different molecules, but they 12 00:00:28,059 --> 00:00:29,379 want to stay close to each other. 13 00:00:29,379 --> 00:00:32,329 There are these little bonds between them and they want to 14 00:00:32,329 --> 00:00:32,939 stay close. 15 00:00:32,939 --> 00:00:37,259 If you increase the average kinetic energy enough, or 16 00:00:37,259 --> 00:00:39,469 essentially increase the temperature enough and then 17 00:00:39,469 --> 00:00:43,810 overcome the heat of fusion, we know that, all of a sudden, 18 00:00:43,810 --> 00:00:46,740 even these bonds aren't strong enough to even keep them 19 00:00:46,740 --> 00:00:50,850 close, and the molecules separate and they get into a 20 00:00:50,850 --> 00:00:52,560 gaseous phase. 21 00:00:52,560 --> 00:00:55,050 And there they have a lot of kinetic energy, and they're 22 00:00:55,049 --> 00:00:59,319 bouncing around, and they take the shape of their container. 23 00:00:59,320 --> 00:01:02,090 But there's an interesting thing to think about. 24 00:01:02,090 --> 00:01:04,709 Temperature is average kinetic energy. 25 00:01:04,709 --> 00:01:11,189 26 00:01:11,189 --> 00:01:15,159 Which implies, and it's true, that all of the molecules do 27 00:01:15,159 --> 00:01:18,939 not have the same kinetic energy. 28 00:01:18,939 --> 00:01:20,329 Let's say even they did. 29 00:01:20,329 --> 00:01:23,310 Then these guys would bump into this guy, and you could 30 00:01:23,310 --> 00:01:25,430 think of them as billiard balls, and they transfer all 31 00:01:25,430 --> 00:01:26,550 of the momentum to this guy. 32 00:01:26,549 --> 00:01:29,289 Now this guy has a ton of kinetic energy. 33 00:01:29,290 --> 00:01:30,870 These guys have a lot less. 34 00:01:30,870 --> 00:01:32,050 This guy has a ton. 35 00:01:32,049 --> 00:01:33,049 These guys have a lot less. 36 00:01:33,049 --> 00:01:35,379 There's a huge distribution of kinetic energy. 37 00:01:35,379 --> 00:01:38,640 If you look at the surface atoms or the surface 38 00:01:38,640 --> 00:01:41,489 molecules, and I care about the surface molecules because 39 00:01:41,489 --> 00:01:46,449 those are the first ones to vaporize or-- I 40 00:01:46,450 --> 00:01:47,269 shouldn't jump the gun. 41 00:01:47,269 --> 00:01:50,339 They're the ones capable of leaving if they had enough 42 00:01:50,340 --> 00:01:51,719 kinetic energy. 43 00:01:51,719 --> 00:01:55,730 If I were to draw a distribution of the surface 44 00:01:55,730 --> 00:01:59,240 molecules-- let me draw a little graph here. 45 00:01:59,239 --> 00:02:07,819 So in this dimension, I have kinetic energy, and on this 46 00:02:07,819 --> 00:02:12,150 dimension, this is just a relative concentration. 47 00:02:12,150 --> 00:02:15,719 And this is just my best estimate, but it should give 48 00:02:15,719 --> 00:02:16,199 you the idea. 49 00:02:16,199 --> 00:02:18,000 So there's some average kinetic energy at some 50 00:02:18,000 --> 00:02:19,360 temperature, right? 51 00:02:19,360 --> 00:02:21,780 This is the average kinetic energy. 52 00:02:21,780 --> 00:02:23,650 And then the kinetic energy of all the parts, it's going to 53 00:02:23,650 --> 00:02:27,090 be a distribution around that, so maybe it looks something 54 00:02:27,090 --> 00:02:30,770 like this: a bell curve. 55 00:02:30,770 --> 00:02:33,200 You could watch the statistics videos to learn more about the 56 00:02:33,199 --> 00:02:34,869 normal distribution, but I think the normal 57 00:02:34,870 --> 00:02:37,515 distribution-- this is supposed to be a normal, so it 58 00:02:37,514 --> 00:02:40,469 just gets smaller and smaller as you go there. 59 00:02:40,469 --> 00:02:43,819 And so at any given time, although the average is here, 60 00:02:43,819 --> 00:02:44,969 there's some molecules that have a 61 00:02:44,969 --> 00:02:46,560 very low kinetic energy. 62 00:02:46,560 --> 00:02:49,590 They're moving slowly or maybe they have-- well, let's just 63 00:02:49,590 --> 00:02:50,960 say they're moving slowly. 64 00:02:50,960 --> 00:02:53,510 And at any given time, you have some molecules that have 65 00:02:53,509 --> 00:02:55,769 a very high kinetic energy, maybe just because of the 66 00:02:55,770 --> 00:02:59,950 random bumps that it gets from other molecules. 67 00:02:59,949 --> 00:03:03,519 It's accrued a lot of velocity or at least a lot of momentum. 68 00:03:03,520 --> 00:03:06,740 So the question arises, are any of these 69 00:03:06,740 --> 00:03:08,680 molecules fast enough? 70 00:03:08,680 --> 00:03:10,819 Do they have enough kinetic energy to escape? 71 00:03:10,819 --> 00:03:12,689 And so there is some kinetic energy. 72 00:03:12,689 --> 00:03:16,520 I'll draw some threshold here, where if you have more than 73 00:03:16,520 --> 00:03:18,710 that amount of kinetic energy, you actually have enough to 74 00:03:18,710 --> 00:03:20,969 escape if you are surface atom. 75 00:03:20,969 --> 00:03:24,520 Now, there could be a dude down here who has a ton of 76 00:03:24,520 --> 00:03:26,050 kinetic energy. 77 00:03:26,050 --> 00:03:28,760 But in order for him to escape, he'd have to bump 78 00:03:28,759 --> 00:03:31,759 through all these other liquid molecules on the way out, so 79 00:03:31,759 --> 00:03:34,229 it's a very-- in fact, he probably won't escape. 80 00:03:34,229 --> 00:03:36,539 It's the surface atoms that we care about because those are 81 00:03:36,539 --> 00:03:38,359 the ones that are interfacing directly with 82 00:03:38,360 --> 00:03:40,310 the pressure outside. 83 00:03:40,310 --> 00:03:42,530 So let's say this is the gas outside. 84 00:03:42,530 --> 00:03:45,520 It's going to be much less dense. 85 00:03:45,520 --> 00:03:48,800 It doesn't have to be, but let's assume it is. 86 00:03:48,800 --> 00:03:52,420 These are the guys that kind of can escape into the air 87 00:03:52,419 --> 00:03:55,149 above it, if we assume that there's some air above it. 88 00:03:55,150 --> 00:03:59,530 So at any given time, there's some fraction of the particles 89 00:03:59,530 --> 00:04:02,469 or the molecules that can escape. 90 00:04:02,469 --> 00:04:04,669 So you're next question is, hey, well, doesn't that mean 91 00:04:04,669 --> 00:04:07,469 that they will be vaporized or they will turn into gas? 92 00:04:07,469 --> 00:04:09,599 And yes, it does. 93 00:04:09,599 --> 00:04:11,479 So at any given time, you have some 94 00:04:11,479 --> 00:04:14,439 molecules that are escaping. 95 00:04:14,439 --> 00:04:17,969 Those molecules-- what it's called is evaporation. 96 00:04:17,970 --> 00:04:19,350 This isn't a foreign concept to you. 97 00:04:19,350 --> 00:04:22,510 If you leave water outside, it will evaporate, even though 98 00:04:22,509 --> 00:04:25,599 outside, hopefully, in your place, is below the boiling 99 00:04:25,600 --> 00:04:27,950 temperature, or the normal boiling temperature of water. 100 00:04:27,949 --> 00:04:30,089 The normal boiling point is just the boiling point at 101 00:04:30,089 --> 00:04:31,560 atmospheric pressure. 102 00:04:31,560 --> 00:04:36,069 If you just leave water out, over time, it will evaporate. 103 00:04:36,069 --> 00:04:39,459 What happens is some of these molecules that have unusually 104 00:04:39,459 --> 00:04:42,819 high kinetic energy do escape. 105 00:04:42,819 --> 00:04:45,990 They do escape, and if you have your pot or pan outside 106 00:04:45,990 --> 00:04:48,605 or, even better, outside of your house, what happens is 107 00:04:48,605 --> 00:04:51,839 they escape, and then the wind blows. 108 00:04:51,839 --> 00:04:54,000 The wind will blow and then blow these guys away. 109 00:04:54,000 --> 00:04:56,009 And then a few more will escape, the wind blows and 110 00:04:56,009 --> 00:04:56,939 blows them all away. 111 00:04:56,939 --> 00:04:59,329 And a few more escape, and the wind blows and blows 112 00:04:59,329 --> 00:05:00,439 them all the way. 113 00:05:00,439 --> 00:05:03,810 So over time, you'll end up with an empty pan that once 114 00:05:03,810 --> 00:05:05,050 held water. 115 00:05:05,050 --> 00:05:07,020 Now, the question is what happens if you 116 00:05:07,019 --> 00:05:09,209 have a closed system? 117 00:05:09,209 --> 00:05:13,199 Well, we've all done that experiment, either on purpose 118 00:05:13,199 --> 00:05:14,979 or inadvertently, leaving something outside and seeing 119 00:05:14,980 --> 00:05:19,009 that the water will evaporate. 120 00:05:19,009 --> 00:05:21,449 What happens in a closed system where there isn't wind 121 00:05:21,449 --> 00:05:22,099 to blow away? 122 00:05:22,100 --> 00:05:27,780 So let me just draw-- there you go. 123 00:05:27,779 --> 00:05:31,919 Let's say a closed system, and I have-- it doesn't have to be 124 00:05:31,920 --> 00:05:34,060 water, but I have some liquid down here. 125 00:05:34,060 --> 00:05:38,720 126 00:05:38,720 --> 00:05:42,350 And there's some pressure from the air above it. 127 00:05:42,350 --> 00:05:44,020 Let's just say it was at atmospheric pressure. 128 00:05:44,019 --> 00:05:44,779 It doesn't have to be. 129 00:05:44,779 --> 00:05:47,709 So there's some air and the air has some 130 00:05:47,709 --> 00:05:50,989 kinetic energy over here. 131 00:05:50,990 --> 00:05:54,199 So, of course, do the water molecules. 132 00:05:54,199 --> 00:05:56,279 And some of them start to evaporate. 133 00:05:56,279 --> 00:05:59,489 So some of the water molecules that are up here in the 134 00:05:59,490 --> 00:06:02,009 distribution, they have enough energy to escape, so they 135 00:06:02,009 --> 00:06:06,889 start hanging out with the air molecules, right? 136 00:06:06,889 --> 00:06:08,050 Now something interesting happens. 137 00:06:08,050 --> 00:06:10,285 This is the distribution of the molecules 138 00:06:10,285 --> 00:06:11,570 in the liquid state. 139 00:06:11,569 --> 00:06:14,349 Well, there's also a distribution of the kinetic 140 00:06:14,350 --> 00:06:16,080 energies of the molecules in the gaseous state. 141 00:06:16,079 --> 00:06:18,649 Just like different things are bumping into each other and 142 00:06:18,649 --> 00:06:21,409 gaining and losing kinetic energy down here, the same 143 00:06:21,410 --> 00:06:22,570 thing is happening up here. 144 00:06:22,569 --> 00:06:25,149 So maybe this guy has a lot of kinetic energy, but he bumps 145 00:06:25,149 --> 00:06:26,529 into stuff and he loses it. 146 00:06:26,529 --> 00:06:28,069 And then he'll come back down. 147 00:06:28,069 --> 00:06:29,939 So there's some set of molecules. 148 00:06:29,939 --> 00:06:32,219 I'll do it in another set of blue. 149 00:06:32,220 --> 00:06:34,450 These are still the water-- or whatever the fluid we're 150 00:06:34,449 --> 00:06:38,000 talking about-- that come back from the vapor state back into 151 00:06:38,000 --> 00:06:39,250 the liquid state. 152 00:06:39,250 --> 00:06:44,439 And so what happens is, there's always a bit of 153 00:06:44,439 --> 00:06:47,670 evaporation and there's always a bit of condensation because 154 00:06:47,670 --> 00:06:48,470 you always have this 155 00:06:48,470 --> 00:06:51,600 distribution of kinetic energies. 156 00:06:51,600 --> 00:06:54,850 At any given moment in time, out of the vapor above the 157 00:06:54,850 --> 00:06:58,055 liquid, some of the vapor loses its kinetic energy and 158 00:06:58,055 --> 00:06:59,560 then it goes back into the liquid state. 159 00:06:59,560 --> 00:07:03,060 Some of the surface liquid gains kinetic energy by random 160 00:07:03,060 --> 00:07:07,050 bumps and whatever else and goes into the vapor state. 161 00:07:07,050 --> 00:07:10,699 And the vapor state will continue to happen until you 162 00:07:10,699 --> 00:07:12,789 get to some type of equilibrium. 163 00:07:12,790 --> 00:07:14,770 And when you get that equilibrium, we're at some 164 00:07:14,769 --> 00:07:15,919 pressure up here. 165 00:07:15,920 --> 00:07:18,520 So let me see, some pressure. 166 00:07:18,519 --> 00:07:22,529 And the pressure is caused by these vapor particles over 167 00:07:22,529 --> 00:07:30,969 here, and that pressure is called the vapor pressure. 168 00:07:30,970 --> 00:07:32,270 I want to make sure you understand this. 169 00:07:32,269 --> 00:07:36,129 So the vapor pressure is the pressure created, and this is 170 00:07:36,129 --> 00:07:45,769 at a given temperature for a given molecule, right? 171 00:07:45,769 --> 00:07:49,459 Every molecule or every type of substance will have a 172 00:07:49,459 --> 00:07:51,810 different vapor pressure at different temperatures, and 173 00:07:51,810 --> 00:07:53,720 obviously every different type of substance will also have 174 00:07:53,720 --> 00:07:55,130 different vapor pressures. 175 00:07:55,129 --> 00:07:56,920 For a given temperature and a given molecule, it's the 176 00:07:56,920 --> 00:08:00,860 pressure at which you have a pressure created by the vapor 177 00:08:00,860 --> 00:08:03,009 molecules where you have an equilibrium. 178 00:08:03,009 --> 00:08:06,310 Where you have just as many things vaporizing as things 179 00:08:06,310 --> 00:08:08,120 going back into the liquid state. 180 00:08:08,120 --> 00:08:10,310 And we learned before that the more pressure you have, the 181 00:08:10,310 --> 00:08:13,780 harder it is to vaporize even more, right? 182 00:08:13,779 --> 00:08:17,529 We learned in the phase state things that if you are at 100 183 00:08:17,529 --> 00:08:20,189 degrees at ultra-high pressure, and you were dealing 184 00:08:20,189 --> 00:08:22,410 with water, you would still be in the liquid state. 185 00:08:22,410 --> 00:08:26,290 So the vapor creates some pressure and it'll keep 186 00:08:26,290 --> 00:08:28,240 happening, depending on how badly this 187 00:08:28,240 --> 00:08:30,319 liquid wants to evaporate. 188 00:08:30,319 --> 00:08:34,629 But it keeps vaporizing until the point that you have just 189 00:08:34,629 --> 00:08:37,308 as much-- I guess you could kind of view it as density up 190 00:08:37,308 --> 00:08:38,779 here, but I don't want to think-- you have just as many 191 00:08:38,779 --> 00:08:41,918 molecules here converting into this state as molecules here 192 00:08:41,918 --> 00:08:43,788 converting into this state. 193 00:08:43,788 --> 00:08:46,419 So just to get an intuition of what vapor pressure is or how 194 00:08:46,419 --> 00:08:48,750 it goes with different molecules, molecules that 195 00:08:48,750 --> 00:08:59,929 really want to evaporate-- and so why would a molecule want 196 00:08:59,929 --> 00:09:00,349 to evaporate? 197 00:09:00,350 --> 00:09:04,350 It could have high kinetic energy, so this would be at a 198 00:09:04,350 --> 00:09:06,529 high temperature. 199 00:09:06,529 --> 00:09:11,689 It could have low intermolecular forces, right? 200 00:09:11,690 --> 00:09:14,620 It could be molecular. 201 00:09:14,620 --> 00:09:17,250 Obviously, the noble gases have very low molecular 202 00:09:17,250 --> 00:09:22,200 forces, but in general, most hydrocarbons or gasoline or 203 00:09:22,200 --> 00:09:24,480 methane or all of these things, they really want to 204 00:09:24,480 --> 00:09:26,570 evaporate because they have much lower intermolecular 205 00:09:26,570 --> 00:09:28,330 forces than, say, water. 206 00:09:28,330 --> 00:09:30,110 Or they could just be light molecules. 207 00:09:30,110 --> 00:09:32,909 208 00:09:32,909 --> 00:09:35,549 You could look at the physics lectures, but kinetic energy 209 00:09:35,549 --> 00:09:38,319 it's a function of mass and velocity. 210 00:09:38,320 --> 00:09:41,830 So you could have a pretty respectable kinetic energy 211 00:09:41,830 --> 00:09:44,879 because you have a high mass and a low velocity. 212 00:09:44,879 --> 00:09:48,480 So if you have a light mass and the same kinetic energy, 213 00:09:48,480 --> 00:09:49,904 you're more likely to have a higher velocity. 214 00:09:49,904 --> 00:09:52,149 You could watch the kinetic energy videos for that. 215 00:09:52,149 --> 00:09:57,009 But something that wants to evaporate, a lot of its 216 00:09:57,009 --> 00:10:00,240 molecules-- let me do it in a different color. 217 00:10:00,240 --> 00:10:03,860 Something that wants to evaporate really bad, a lot 218 00:10:03,860 --> 00:10:07,129 more of its molecules will have to enter into this vapor 219 00:10:07,129 --> 00:10:12,889 state in order for the equilibrium to be reached. 220 00:10:12,889 --> 00:10:15,370 Let me do it all in the same color. 221 00:10:15,370 --> 00:10:20,810 So the pressure created by its evaporated molecules is going 222 00:10:20,809 --> 00:10:23,899 to be higher for it to get to that equilibrium state, so it 223 00:10:23,899 --> 00:10:25,394 has high vapor pressure. 224 00:10:25,394 --> 00:10:34,019 225 00:10:34,019 --> 00:10:36,539 And on the other side, if you're at a low temperature or 226 00:10:36,539 --> 00:10:40,159 you have strong intermolecular forces or you have a heavy 227 00:10:40,159 --> 00:10:44,449 molecule, then you're going to have a low vapor pressure. 228 00:10:44,450 --> 00:10:50,490 For example, iron has a very low vapor pressure because 229 00:10:50,490 --> 00:10:54,220 it's not vaporizing while-- let me think of something. 230 00:10:54,220 --> 00:10:56,450 Carbon dioxide has a relatively much 231 00:10:56,450 --> 00:10:57,620 higher vapor pressure. 232 00:10:57,620 --> 00:11:00,460 Much more of carbon dioxide is going to evaporate 233 00:11:00,460 --> 00:11:01,300 when you have it. 234 00:11:01,299 --> 00:11:02,769 Well, I really shouldn't use that because you're going 235 00:11:02,769 --> 00:11:05,139 straight from the liquid to the solid state, but I think 236 00:11:05,139 --> 00:11:06,110 you get the idea. 237 00:11:06,110 --> 00:11:09,060 And something that has a high vapor pressure, that wants to 238 00:11:09,059 --> 00:11:15,119 evaporate really bad, we say it has a high volatility. 239 00:11:15,120 --> 00:11:16,620 You've probably heard that word before. 240 00:11:16,620 --> 00:11:23,850 241 00:11:23,850 --> 00:11:28,389 So, for example, gasoline has a higher-- it's more volatile 242 00:11:28,389 --> 00:11:31,059 than water, and that's why it evaporates, and it also has a 243 00:11:31,059 --> 00:11:32,229 higher vapor pressure. 244 00:11:32,230 --> 00:11:34,779 Because if you were to put it in a closed container, more 245 00:11:34,779 --> 00:11:39,169 gasoline at the same temperature and the same 246 00:11:39,169 --> 00:11:42,120 atmospheric pressure, will enter into the vapor state. 247 00:11:42,120 --> 00:11:46,990 And so that vapor state will generate more pressure to 248 00:11:46,990 --> 00:11:53,330 offset the natural inclination of the gasoline to want to 249 00:11:53,330 --> 00:11:55,750 escape than in the case with water. 250 00:11:55,750 --> 00:11:59,340 Now, an interesting thing happens when this vapor 251 00:11:59,340 --> 00:12:02,700 pressure is equal to the atmospheric pressure. 252 00:12:02,700 --> 00:12:05,990 So right now, this is our closed container and you have 253 00:12:05,990 --> 00:12:08,370 the atmosphere here at a certain pressure. 254 00:12:08,370 --> 00:12:11,789 Let's say until now, we've assumed that the atmosphere 255 00:12:11,789 --> 00:12:14,539 was at a higher pressure, for the most part keeping these 256 00:12:14,539 --> 00:12:16,009 molecules contained. 257 00:12:16,009 --> 00:12:18,439 Maybe some atmosphere molecules are coming in here, 258 00:12:18,440 --> 00:12:21,740 and maybe some of the vapor molecules are escaping a bit, 259 00:12:21,740 --> 00:12:23,820 but it's keeping it contained because this is at a higher 260 00:12:23,820 --> 00:12:27,020 pressure out here than this vapor pressure. 261 00:12:27,019 --> 00:12:30,490 And of course the pressure right here, at the surface of 262 00:12:30,490 --> 00:12:32,700 the molecule, is going to be the combination of the partial 263 00:12:32,700 --> 00:12:35,900 pressure due to the few atmospheric molecules that 264 00:12:35,899 --> 00:12:38,059 come in, plus the vapor pressure. 265 00:12:38,059 --> 00:12:41,479 But once that vapor pressure becomes equal to that 266 00:12:41,480 --> 00:12:45,060 atmospheric pressure, so it can press out with the same 267 00:12:45,059 --> 00:12:48,019 amount of force-- you can kind of view it as force per area-- 268 00:12:48,019 --> 00:12:51,259 so then the molecules can start to escape. 269 00:12:51,259 --> 00:12:53,490 It can push the atmosphere back. 270 00:12:53,490 --> 00:12:55,600 And so you start having a gap here. 271 00:12:55,600 --> 00:12:59,529 You start having a vacuum. 272 00:12:59,529 --> 00:13:01,409 I don't want to use exactly a vacuum, but since the 273 00:13:01,409 --> 00:13:04,610 molecules escaped, more and more of these molecules can 274 00:13:04,610 --> 00:13:05,279 start going out. 275 00:13:05,279 --> 00:13:07,559 And at that point, you've reached the boiling point of 276 00:13:07,559 --> 00:13:10,579 the substance when the vapor pressure is equal to the 277 00:13:10,580 --> 00:13:12,250 atmospheric pressure. 278 00:13:12,250 --> 00:13:15,759 Just to get a sense of what all of this means, let's look 279 00:13:15,759 --> 00:13:18,299 at the vapor pressure for water. 280 00:13:18,299 --> 00:13:22,539 This is water right here, H2O. 281 00:13:22,539 --> 00:13:25,639 I should do that in black. 282 00:13:25,639 --> 00:13:31,189 And so you see at 760-- so atmospheric pressure, we're in 283 00:13:31,190 --> 00:13:35,185 torr now, but that's just a different-- 760 torr is equal 284 00:13:35,184 --> 00:13:38,309 to 1 atmosphere, so that's about right. 285 00:13:38,309 --> 00:13:41,609 That's about right there, so it's 1 atmosphere. 286 00:13:41,610 --> 00:13:46,139 So at atmospheric pressure, the vapor pressure at 100 287 00:13:46,139 --> 00:13:50,519 degrees Celsius for water-- the vapor is at 100 degrees 288 00:13:50,519 --> 00:13:51,370 Celsius for water. 289 00:13:51,370 --> 00:13:54,330 Or I guess another way to put it, at 100 degrees Celsius, 290 00:13:54,330 --> 00:14:00,259 you have 760 torr of vapor pressure, which is exactly the 291 00:14:00,259 --> 00:14:03,819 atmospheric pressure, or 1 atmosphere, at sea level. 292 00:14:03,820 --> 00:14:08,070 So at 100 degrees, vapor pressure is equal to 293 00:14:08,070 --> 00:14:10,185 atmospheric, or sea level atmospheric. 294 00:14:10,184 --> 00:14:12,769 295 00:14:12,769 --> 00:14:15,500 And so you're going to boil, which we all know is true. 296 00:14:15,500 --> 00:14:19,970 And then at lower temperatures, your vapor 297 00:14:19,970 --> 00:14:22,850 pressure is going to be lower than the 298 00:14:22,850 --> 00:14:24,370 atmospheric pressure, right? 299 00:14:24,370 --> 00:14:26,779 Let's see, here it looks like 300 something. 300 00:14:26,779 --> 00:14:27,649 But then what happens? 301 00:14:27,649 --> 00:14:30,590 If you lowered the atmospheric pressure enough, if you were 302 00:14:30,590 --> 00:14:34,370 to pump air out of the container, or whatever, low 303 00:14:34,370 --> 00:14:37,950 enough, so if you brought the atmospheric pressure down to 304 00:14:37,950 --> 00:14:40,620 this vapor pressure, then again, you will have boiling. 305 00:14:40,620 --> 00:14:43,389 And we saw that in the phase change diagrams, that you can 306 00:14:43,389 --> 00:14:46,480 boil something at a lower temperature if you lower the 307 00:14:46,480 --> 00:14:47,710 atmospheric pressure. 308 00:14:47,710 --> 00:14:49,660 And that's because you're lowering the atmospheric 309 00:14:49,659 --> 00:14:54,069 pressure to the vapor pressure of the substance. 310 00:14:54,070 --> 00:14:56,360 And here's a comparative chart, and this is 311 00:14:56,360 --> 00:14:56,789 interesting. 312 00:14:56,789 --> 00:14:59,669 You see this is kind of an exponential increase with 313 00:14:59,669 --> 00:15:03,269 temperature of vapor pressure. 314 00:15:03,269 --> 00:15:05,759 And that's because, if you think about that distribution 315 00:15:05,759 --> 00:15:08,850 we did before, this is at one kinetic energy. 316 00:15:08,850 --> 00:15:11,659 If you increase the amount of kinetic energy, then your 317 00:15:11,659 --> 00:15:13,100 distribution will look like this. 318 00:15:13,100 --> 00:15:14,620 The temperature has gone up. 319 00:15:14,620 --> 00:15:16,769 And now you have a lot, lot more. 320 00:15:16,769 --> 00:15:17,649 It's not just linear. 321 00:15:17,649 --> 00:15:20,889 You have a lot more particles that can now escape and have 322 00:15:20,889 --> 00:15:24,620 the kinetic energy to evaporate. 323 00:15:24,620 --> 00:15:26,720 And you can see it's this exponential increase as you 324 00:15:26,720 --> 00:15:28,029 increase the temperature. 325 00:15:28,029 --> 00:15:29,059 Now, here's another chart. 326 00:15:29,059 --> 00:15:30,989 You say, hey, where's that exponential increase going? 327 00:15:30,990 --> 00:15:32,990 That's because this is a logarithmic chart. 328 00:15:32,990 --> 00:15:35,879 You can see the scale. 329 00:15:35,879 --> 00:15:41,110 It increases exponentially as opposed to linearly, so it 330 00:15:41,110 --> 00:15:44,529 goes from 0.1 to 10, so equal distances are actually up by a 331 00:15:44,529 --> 00:15:46,009 factor of 10, so that's why you don't see 332 00:15:46,009 --> 00:15:47,600 that logarithmic move. 333 00:15:47,600 --> 00:15:49,860 But these are just for different substances. 334 00:15:49,860 --> 00:15:53,389 Propane, you see at any given-- so let's go at like a 335 00:15:53,389 --> 00:15:54,159 decent temperature. 336 00:15:54,159 --> 00:15:56,471 Let's go 20 degrees Celsius. 337 00:15:56,471 --> 00:16:02,939 At 20 Celsius, propane has the highest vapor pressure. 338 00:16:02,940 --> 00:16:08,310 So this is 1 atmosphere, so propane will actually 339 00:16:08,309 --> 00:16:11,614 evaporate, will actually boil at 20 degrees Celsius. 340 00:16:11,615 --> 00:16:13,440 It will actually completely boil and go 341 00:16:13,440 --> 00:16:14,520 into the gaseous state. 342 00:16:14,519 --> 00:16:17,409 Because its vapor pressure is so much higher than 343 00:16:17,409 --> 00:16:20,469 atmospheric pressure, if we're assuming we're at sea level. 344 00:16:20,470 --> 00:16:22,910 And you could do that for different molecules. 345 00:16:22,909 --> 00:16:24,730 Methyl chloride is the next one. 346 00:16:24,730 --> 00:16:26,430 It's a slightly lower vapor pressure, 347 00:16:26,429 --> 00:16:28,250 but still very volatile. 348 00:16:28,250 --> 00:16:31,659 It would still definitely boil and turn into the gaseous 349 00:16:31,659 --> 00:16:34,389 state at 20 degrees Celsius if we're at sea level because sea 350 00:16:34,389 --> 00:16:36,330 level is right there. 351 00:16:36,330 --> 00:16:39,150 Let's see, at sea level, if you wanted to keep something-- 352 00:16:39,149 --> 00:16:41,750 so sea level is this pressure-- if you wanted to 353 00:16:41,750 --> 00:16:46,009 keep let's say, methyl chloride. 354 00:16:46,009 --> 00:16:49,980 If you wanted to keep methyl chloride in the liquid state, 355 00:16:49,980 --> 00:16:52,170 or in equilibrium with the liquid state instead of 356 00:16:52,169 --> 00:16:53,579 boiling, you would have to be at least at 357 00:16:53,580 --> 00:16:54,240 around-- what is this? 358 00:16:54,240 --> 00:16:58,009 Minus 25 degrees Celsius in order for that. 359 00:16:58,009 --> 00:17:00,429 Propane, even at minus 25 degrees is still in the 360 00:17:00,429 --> 00:17:03,289 gaseous state because its vapor 361 00:17:03,289 --> 00:17:05,480 pressure is still higher. 362 00:17:05,480 --> 00:17:10,140 And then, of course, if you have butane, for example. 363 00:17:10,140 --> 00:17:17,720 Butane I think is what they put in lighters, but butane 364 00:17:17,720 --> 00:17:21,549 will be in the liquid state as long as you're at around 365 00:17:21,549 --> 00:17:22,720 roughly 0 degrees. 366 00:17:22,720 --> 00:17:24,559 In a lighter, you might say, oh, it's in a liquid state. 367 00:17:24,559 --> 00:17:27,139 They probably increase the pressure so the pressure in 368 00:17:27,140 --> 00:17:28,680 the lighter is probably something higher. 369 00:17:28,680 --> 00:17:33,930 Maybe it's at 2 atmospheres or something, so that the butane 370 00:17:33,930 --> 00:17:35,940 at room temperature will stay in the liquid state. 371 00:17:35,940 --> 00:17:36,220 Who knows? 372 00:17:36,220 --> 00:17:38,160 I don't know what the pressure is in there. 373 00:17:38,160 --> 00:17:39,930 This is just an interesting chart to look at, that there's 374 00:17:39,930 --> 00:17:41,690 actually a bunch of different vapor pressures. 375 00:17:41,690 --> 00:17:44,870 You can see at atmospheric pressure what's likely to be a 376 00:17:44,869 --> 00:17:47,059 gas or a liquid at different temperatures, and then you 377 00:17:47,059 --> 00:17:50,309 could see at different temperatures, which are the 378 00:17:50,309 --> 00:17:52,440 things that are most volatile and how much do you have to 379 00:17:52,440 --> 00:17:56,000 increase or decrease the pressure to evaporate 380 00:17:56,000 --> 00:17:57,279 something or to boil it. 381 00:17:57,279 --> 00:17:58,819 Anyway, hopefully you found that useful. 382 00:17:58,819 --> 00:18:01,259 Vapor pressure is something that we encounter every day, 383 00:18:01,259 --> 00:18:03,660 and I'll see you in the next video.