WEBVTT 00:00:06.738 --> 00:00:10.329 Why does salt dissolve in water but oil doesn't? 00:00:10.329 --> 00:00:12.124 Well, in a word, chemistry, 00:00:12.124 --> 00:00:14.328 but that's not very satisfying, is it? 00:00:14.328 --> 00:00:17.442 Well, the reason salt dissolves and oil does not 00:00:17.442 --> 00:00:19.152 comes down to the two big reasons 00:00:19.152 --> 00:00:21.348 why anything happens at all: 00:00:21.348 --> 00:00:22.281 energetics 00:00:22.281 --> 00:00:23.489 and entropy. 00:00:23.489 --> 00:00:25.322 Energetics deals primarily 00:00:25.322 --> 00:00:28.203 with the attractive forces between things. 00:00:28.203 --> 00:00:31.155 When we look at oil or salt in water, 00:00:31.155 --> 00:00:34.152 we focus on the forces between particles 00:00:34.152 --> 00:00:36.779 on a very, very, very small scale, 00:00:36.779 --> 00:00:38.180 the molecular level. 00:00:38.180 --> 00:00:39.898 To give you a sense of this scale, 00:00:39.898 --> 00:00:41.599 in one glass of water, 00:00:41.599 --> 00:00:43.110 there are more molecules 00:00:43.110 --> 00:00:45.816 than known stars in the universe. 00:00:45.816 --> 00:00:49.448 Now, all of these molecules are in constant motion, 00:00:49.448 --> 00:00:52.460 moving, vibrating, and rotating. 00:00:52.460 --> 00:00:54.774 What prevents almost all of those molecules 00:00:54.774 --> 00:00:56.858 from just flying out of the glass 00:00:56.858 --> 00:01:00.027 are the attractive interactions between molecules. 00:01:00.027 --> 00:01:01.230 The strength of the interactions 00:01:01.230 --> 00:01:04.482 between water, itself, and other substances 00:01:04.482 --> 00:01:07.733 is what we mean when we say energetics. 00:01:07.733 --> 00:01:09.874 You can think of the water molecules engaging 00:01:09.874 --> 00:01:11.419 in a constant dance, 00:01:11.419 --> 00:01:13.047 sort of like a square dance 00:01:13.047 --> 00:01:16.501 where they constantly and randomly exchange partners. 00:01:16.501 --> 00:01:19.185 Put simply, the ability for substances 00:01:19.185 --> 00:01:20.676 to interact with water, 00:01:20.676 --> 00:01:22.298 balanced with how they disrupt 00:01:22.298 --> 00:01:24.881 how water interacts with itself, 00:01:24.881 --> 00:01:27.230 plays an important role in explaining 00:01:27.230 --> 00:01:29.899 why certain things mix well into water 00:01:29.899 --> 00:01:31.649 and others don't. 00:01:32.048 --> 00:01:33.912 Entropy basically describes 00:01:33.912 --> 00:01:36.631 the way things and energy can be arranged 00:01:36.631 --> 00:01:38.503 based on random motion. 00:01:38.503 --> 00:01:41.200 For example, think of the air in a room. 00:01:41.200 --> 00:01:44.062 Imagine all the different possible arrangements 00:01:44.062 --> 00:01:46.650 in space for the trillions of particles 00:01:46.650 --> 00:01:48.131 that make up the air. 00:01:48.131 --> 00:01:49.167 Some of those arrangments 00:01:49.167 --> 00:01:52.163 might have all the oxygen molecules over here 00:01:52.163 --> 00:01:54.814 and all the nitrogen molecules over there, 00:01:54.814 --> 00:01:55.855 separated. 00:01:55.855 --> 00:01:58.023 But far more of those possible arrangments 00:01:58.023 --> 00:02:01.023 have those molecules mixed up with one another. 00:02:01.023 --> 00:02:03.481 So, entropy favors mixing. 00:02:03.481 --> 00:02:06.334 Energetics deals with attractive forces. 00:02:06.334 --> 00:02:08.417 And so, if attractive forces are present, 00:02:08.417 --> 00:02:10.321 the probability of some arrangements 00:02:10.321 --> 00:02:11.500 can be enhanced, 00:02:11.500 --> 00:02:13.780 the ones where things are attracted to each other. 00:02:13.780 --> 00:02:16.294 So, it is always the balance of these two things 00:02:16.294 --> 00:02:18.871 that determines what happens. 00:02:18.871 --> 00:02:20.285 On the molecular level, 00:02:20.285 --> 00:02:22.664 water is comprised of water molecules, 00:02:22.664 --> 00:02:25.829 made up of two hydrogen atoms and an oxygen atom. 00:02:25.829 --> 00:02:28.497 As liquid water, these molecules are engaged 00:02:28.497 --> 00:02:30.831 in a constant and random square dance 00:02:30.831 --> 00:02:34.386 that is called the hydrogen-bonding network. 00:02:34.386 --> 00:02:35.771 Entropy favors keeping 00:02:35.771 --> 00:02:38.018 the square dance going at all times. 00:02:38.018 --> 00:02:39.314 There are always more ways 00:02:39.314 --> 00:02:41.142 that all the water molecules can arrange 00:02:41.142 --> 00:02:42.349 in a square dance, 00:02:42.349 --> 00:02:44.079 as compared to if the water molecules 00:02:44.079 --> 00:02:45.421 did a line dance. 00:02:45.421 --> 00:02:47.781 So, the square dance constantly goes on. 00:02:47.781 --> 00:02:50.750 So, what happens when you put salt in the water? 00:02:50.750 --> 00:02:52.748 Well, on the molecular level, 00:02:52.748 --> 00:02:55.584 salt is actually made up of two different ions, 00:02:55.584 --> 00:02:57.838 chlorine and sodium, 00:02:57.838 --> 00:03:00.082 that are organized like a brick wall. 00:03:00.082 --> 00:03:01.502 They show up to the dance 00:03:01.502 --> 00:03:03.386 as a big group in formation 00:03:03.386 --> 00:03:05.040 and sit on the side at first, 00:03:05.040 --> 00:03:07.431 shy and a bit reluctant to break apart 00:03:07.431 --> 00:03:10.272 into individual ions to join the dance. 00:03:10.272 --> 00:03:12.456 But secretly, those shy dancers 00:03:12.456 --> 00:03:15.009 just want someone to ask them to join. 00:03:15.009 --> 00:03:18.233 So, when a water randomly bumps into one of them 00:03:18.233 --> 00:03:20.817 and pulls them into the dance away from their group, 00:03:20.817 --> 00:03:21.983 they go. 00:03:21.983 --> 00:03:23.362 And once they go into the dance, 00:03:23.362 --> 00:03:24.948 they don't come back out. 00:03:24.948 --> 00:03:27.352 And in fact, the addition of the salt ions 00:03:27.352 --> 00:03:29.412 adds more possible dance positions 00:03:29.412 --> 00:03:30.790 in the square dance, 00:03:30.790 --> 00:03:33.973 so it is favored for them to stay dancing with water. 00:03:35.156 --> 00:03:37.119 Now, let's take oil. 00:03:37.119 --> 00:03:39.800 With oil, the molecules are sort of interested 00:03:39.800 --> 00:03:40.887 in dancing with water, 00:03:40.887 --> 00:03:43.138 so entropy favors them joining the dance. 00:03:43.138 --> 00:03:45.193 The problem is that oil molecules 00:03:45.193 --> 00:03:47.471 are wearing gigantic ballgowns, 00:03:47.471 --> 00:03:50.040 and they're way bigger than water molecules. 00:03:50.040 --> 00:03:52.474 So, when an oil molecule gets pulled in, 00:03:52.474 --> 00:03:55.299 their size is really disruptive to the dance 00:03:55.299 --> 00:03:56.919 and the random exchange of partners 00:03:56.919 --> 00:03:58.622 that the waters engage in, 00:03:58.622 --> 00:04:00.329 a very important part of the dance. 00:04:00.329 --> 00:04:02.885 In addition, they are not great dancers. 00:04:02.885 --> 00:04:04.805 The water molecules try to engage 00:04:04.805 --> 00:04:06.464 the oil molecules in the dance, 00:04:06.464 --> 00:04:08.890 but they just keep bumping into their dresses 00:04:08.890 --> 00:04:11.055 and taking up all the room on the dance floor. 00:04:11.055 --> 00:04:13.723 There are way more ways the waters can dance 00:04:13.723 --> 00:04:15.765 when the oil gets off the floor, 00:04:15.765 --> 00:04:18.214 so the waters squeeze out the oil, 00:04:18.214 --> 00:04:20.803 pushing it back to the bench with the others. 00:04:20.803 --> 00:04:22.887 Pretty soon, when a large number of oils 00:04:22.887 --> 00:04:24.971 have been squeezed over to the side, 00:04:24.971 --> 00:04:27.028 they band together to commiserate 00:04:27.028 --> 00:04:29.455 about how unfair the waters are being 00:04:29.455 --> 00:04:31.408 and stick together as a group. 00:04:31.408 --> 00:04:32.746 So, it is this combination 00:04:32.746 --> 00:04:35.082 of the interactions between molecules 00:04:35.082 --> 00:04:37.472 and the configurations available to them 00:04:37.472 --> 00:04:38.890 when they're moving randomly 00:04:38.890 --> 00:04:41.216 that dictates whether they mix. 00:04:41.216 --> 00:04:43.637 In other words, water and oil don't mix 00:04:43.637 --> 00:04:47.217 because they just don't make great dance partners.