WEBVTT 00:00:00.280 --> 00:00:04.500 Molecules! So many of them in their infinite and beautiful variety, 00:00:04.500 --> 00:00:08.109 but while that variety is great, it can also be pretty dang overwhelming. 00:00:08.109 --> 00:00:14.989 And so, in order to help this complicated chemical world make a little more sense, we classify and we categorize. 00:00:14.989 --> 00:00:17.520 It's our nature as humans, and it's extremely useful. 00:00:17.520 --> 00:00:21.990 One of the most important of those classifications is whether a molecule is polar or non-polar. 00:00:21.990 --> 00:00:25.250 It's a kind of symmetry, not just of the molecule, but of the charge. 00:00:25.250 --> 00:00:27.710 It's pretty easy to see when you're just lookin' at 'em. 00:00:27.710 --> 00:00:33.260 You got polar and non-polar, polar, non-polar, polar, non-polar. 00:00:33.260 --> 00:00:35.670 I'm gonna take sides right now. I'm on team polar. 00:00:35.670 --> 00:00:40.510 I think polar molecules are way more interesting, despite their wonky, off-balance selves. 00:00:40.510 --> 00:00:43.190 Non-polar molecules are useful, and their symmetry has a kind of beauty, 00:00:43.190 --> 00:00:45.990 but polar, in my humble opinion, is where it's at. 00:00:45.990 --> 00:00:55.160 [Theme Music] 00:00:55.160 --> 00:00:58.479 All right. Now here are two very different types of chemicals. 00:00:58.479 --> 00:01:03.199 Right here I have a stick of butter, and then in this bowl, that's just normal water. 00:01:03.200 --> 00:01:08.660 So I'm just gonna go ahead and squeeze this butter, which if you're wondering is both a terrible and wonderful feeling. 00:01:08.660 --> 00:01:13.140 And then I'm going to [laughs] just drop that. 00:01:13.140 --> 00:01:16.620 Now I'm going to attempt to wash that butter off my hand. 00:01:16.620 --> 00:01:21.200 But that is just not hap... that's just, it's not going anywhere, ever. 00:01:21.200 --> 00:01:23.900 Ever. It's just beading up on me. 00:01:23.900 --> 00:01:29.740 Why? Because water is a polar molecule, and the various chemicals that make up butter are non-polar, 00:01:29.740 --> 00:01:33.440 and water wants nothing to do with that. 00:01:33.440 --> 00:01:37.450 So. What makes a molecule polar? Well, two things. 00:01:37.450 --> 00:01:40.710 First, asymmetrical electron distribution around the molecule. 00:01:40.710 --> 00:01:44.030 You can't have a polar molecule made up entirely of the same element 00:01:44.030 --> 00:01:46.970 because those atoms will all have the same electronegativity, 00:01:46.970 --> 00:01:50.650 and thus the electron distribution will be completely symmetrical. 00:01:50.650 --> 00:01:53.910 Electronegativity is usually thought of as how much an element wants electrons around it, 00:01:53.910 --> 00:01:57.810 but I think it's more about how much electrons want to be near that element. 00:01:57.810 --> 00:02:02.060 If electrons were 13-year-old girls, fluorine would be Niall Horan. 00:02:02.060 --> 00:02:07.670 They'll do anything just to be near it. Why? Some simple periodic trends. 00:02:07.670 --> 00:02:11.940 Electronegativity increases from left to right because there are more protons in the atoms, 00:02:11.940 --> 00:02:14.990 and more protons means more boys in the band. 00:02:14.990 --> 00:02:18.050 Meanwhile, it decreases as you move from top to bottom 00:02:18.050 --> 00:02:23.180 because as the crowd of electrons gets bigger, they start to shield each other from the effects of the protons. 00:02:23.180 --> 00:02:25.720 What I'm trying to say is that electrons are hipsters. 00:02:25.730 --> 00:02:29.130 If a bunch of other electrons are into that thing, they're less interested. 00:02:29.130 --> 00:02:30.850 Now there are a number of other factors here, 00:02:30.850 --> 00:02:34.040 but just like the relationship between tweens and their latest boy band fixation, 00:02:34.040 --> 00:02:37.050 it's complicated and weird and you probably don't want to think too much about it. 00:02:37.050 --> 00:02:39.380 But in this nice little map, you can see that the trend is pretty clear. 00:02:39.380 --> 00:02:42.740 The upper-right is where all the superstars of electro-fame are. 00:02:42.740 --> 00:02:47.240 Oxygen, nitrogen, fluorine, chlorine, and bromine are basically the One Direction of the periodic table. 00:02:47.240 --> 00:02:51.720 So for polarity to occur in a molecule, you have to have two different elements at a minimum, 00:02:51.720 --> 00:02:55.700 and the difference between their electronegativities has to be 0.5 or greater. 00:02:55.700 --> 00:03:00.460 If that's the case, the outer electrons spend enough extra time around the element that's more electronegative 00:03:00.460 --> 00:03:02.860 that chemists label the molecule polar. 00:03:02.860 --> 00:03:06.400 The result is a partially negative charge on the more electronegative part of the molecule 00:03:06.400 --> 00:03:09.540 and a partially positive charge on the less electronegative side. 00:03:09.540 --> 00:03:12.599 Now in extreme cases, like if the electronegativity is greater than 1.6, 00:03:12.599 --> 00:03:16.020 then we end up with two ions in the same molecule. 00:03:16.020 --> 00:03:19.160 This isn't what we're talking about here when we talk about polar molecules. 00:03:19.160 --> 00:03:22.410 We're talking about differences between 0.5 and 1.6. 00:03:22.410 --> 00:03:26.130 Another requirement for polarity: you gotta have geometrical asymmetry. 00:03:26.130 --> 00:03:31.430 CO2 here has the charge asymmetry locked up, but because the molecule is linear, in a straight 00:03:31.430 --> 00:03:34.209 line, it's a kind of symmetrical asymmetry. 00:03:34.209 --> 00:03:39.209 The same thing does for CH4 with its tetrahedron of weakly electronegative hydrogens around 00:03:39.209 --> 00:03:41.180 a more strongly electronegative carbon. 00:03:41.180 --> 00:03:45.380 These molecules have polar bonds, but the molecules themselves are not polar 00:03:45.380 --> 00:03:49.319 because the symmetry of the bonds cancels out the asymmetry of the charges. 00:03:49.319 --> 00:03:53.550 In order for a molecule to be polar, there has to be a dipole moment, 00:03:53.550 --> 00:03:58.480 a separation of the charge around the molecule into a more positive area and a more negative area. 00:03:58.480 --> 00:04:02.560 Lots of molecules are asymmetrical in both electronegativity and geometry. 00:04:02.569 --> 00:04:06.760 Those are our polar molecules, the asymmetrical beauties of chemistry. 00:04:06.760 --> 00:04:09.140 Look at 'em all! They're so quirky and weird! 00:04:09.140 --> 00:04:12.879 We've also got a system for indicating where their charges are. 00:04:12.879 --> 00:04:17.940 We draw an arrow with a plus sign at the tail pointing toward the negative side of the molecule. 00:04:17.940 --> 00:04:22.500 A little lowercase delta plus (δ+) or delta minus (δ–) by the individual atoms signify 00:04:22.500 --> 00:04:25.120 a partial positive pr partial negative charge. 00:04:25.120 --> 00:04:27.530 Liquids made up of polar molecules are really good at 00:04:27.530 --> 00:04:31.710 dissolving solids that are composed of polar or ionic compounds. 00:04:31.710 --> 00:04:34.340 Ionic solids are basically just polarity taken to the extreme, 00:04:34.340 --> 00:04:38.060 so far that instead of having a partial positive and partial negative dipole moment, 00:04:38.060 --> 00:04:41.389 the electrons have completely transferred, creating two charged ions. 00:04:41.389 --> 00:04:44.180 Now I assume we've all heard that like dissolves like, 00:04:44.180 --> 00:04:48.620 so the easiest way to figure out if a liquid is polar or non-polar is just to dump it in some water. 00:04:48.620 --> 00:04:52.960 But the why of this phenomenon is usually just totally glossed over. 00:04:52.970 --> 00:04:55.590 What's actually happening to those molecules? 00:04:55.590 --> 00:05:00.870 It seems like they're all just bigots, terrified of anything a little bit different than themselves. 00:05:00.870 --> 00:05:04.720 But this is chemistry, so there must be some fundamental reason. 00:05:04.720 --> 00:05:09.940 And if it's fundamental, it probably has something to do with decreasing the energy of the system. 00:05:09.940 --> 00:05:11.100 And indeed it does. 00:05:11.100 --> 00:05:14.000 Those partial positive and partial negative charges of water? 00:05:14.000 --> 00:05:17.920 They're at their lowest energy state when they're lining up together, positive to negative, 00:05:17.920 --> 00:05:19.960 into a kind of liquid crystal. 00:05:19.960 --> 00:05:23.120 There's an arrangement there. It flows, of course, 00:05:23.120 --> 00:05:27.840 but the oxygen sides are always doing their best to orient themselves toward the hydrogen 00:05:27.840 --> 00:05:29.030 sides of other molecules. 00:05:29.030 --> 00:05:31.550 You can even see the effects of that attraction 00:05:31.550 --> 00:05:37.640 as the surface tension that allows me to pour more than 100 milliliters of water into a 100 mil container. 00:05:37.640 --> 00:05:43.920 The strength of that surface tension depends on the intermolecular forces that pull molecules of a liquid together. 00:05:43.920 --> 00:05:48.760 These attractive, also called cohesive, forces pull the surface molecules inward. 00:05:48.770 --> 00:05:53.860 And what you see when you look at this pile of water is the result of those cohesive forces, 00:05:53.860 --> 00:05:56.460 minimized surface area in the water in this beaker. 00:05:56.460 --> 00:06:00.229 When you pit a bit of oil into that mix, the water totally freaks out. 00:06:00.229 --> 00:06:03.660 Oils have notoriously non-polar molecules, so suddenly 00:06:03.660 --> 00:06:09.560 there's this mass of uncharged gunk interfering with the nice, orderly arrangement of polar water molecules. 00:06:09.560 --> 00:06:14.840 But if you take a closer look, the processes are very similar to those between water and air. 00:06:14.840 --> 00:06:20.760 Water does everything it can to minimize its surface area and kind of expels the oil droplets. 00:06:20.760 --> 00:06:26.940 Rather than the water disliking the oil, it actually just likes itself much more, so it won't mix with the oil. 00:06:26.949 --> 00:06:29.770 Now if you put polar stuff in, water is all about that, 00:06:29.770 --> 00:06:34.229 and those polar water molecules just go after whatever other partial charges they can find. 00:06:34.229 --> 00:06:36.630 Or, in the case of many ionic solids, 00:06:36.630 --> 00:06:41.139 the partial negative charges on the oxygen side all gang up on the positive ions, 00:06:41.139 --> 00:06:44.910 while the partial positives on the hydrogen side surround the negative ions, 00:06:44.910 --> 00:06:49.080 breaking the crystals apart and dissolving them into freely moving ions. 00:06:49.080 --> 00:06:53.070 In some cases we can actually witness these interactions in unexpected ways. 00:06:53.070 --> 00:06:59.960 Mix 50 milliliters of water with 50 mils of alcohol and what the heck? There's less than 100 mils of liquid! 00:06:59.960 --> 00:07:05.200 The arrangement of water mixed with alcohol is actually more structured, and thus more dense, 00:07:05.200 --> 00:07:06.979 resulting in a smaller volume. 00:07:06.979 --> 00:07:12.720 The polarity of water also results in a phenomenon that makes life possible: hydrogen bonding. 00:07:12.720 --> 00:07:17.160 The partially negative oxygen and positive hydrogen atoms in a water molecule are not 00:07:17.160 --> 00:07:19.479 100% faithful to each other. 00:07:19.480 --> 00:07:25.020 They engage in additional kind of loose relationships with other neighboring hydrogen and oxygen atoms. 00:07:25.020 --> 00:07:29.039 These loose, somewhat fleeting relationships are called hydrogen bonds. 00:07:29.039 --> 00:07:33.499 In ice, 100% of O and H atoms are involved in hydrogen bonding. 00:07:33.500 --> 00:07:37.220 The most energetically favorable spatial arrangement of these bonds 00:07:37.229 --> 00:07:40.759 actually pushes the water molecules apart a bit, 00:07:40.759 --> 00:07:45.550 resulting in the volume of ice being 10% larger than the volume of water, 00:07:45.550 --> 00:07:47.850 which is really weird for solids and liquids. 00:07:47.850 --> 00:07:52.610 When ice melts, there are still about 80% of Os and Hs engaged in hydrogen bonding, 00:07:52.610 --> 00:07:57.289 creating ice-like clusters that keep the volume of the cold water relatively high. 00:07:57.289 --> 00:07:59.220 With rising temperatures, these clusters disappear, 00:07:59.220 --> 00:08:04.700 while the volume of the truly liquid water rises resulting in a major characteristic of water: 00:08:04.700 --> 00:08:07.759 having its highest density at 4 °C. 00:08:07.760 --> 00:08:13.960 And yes, that's why ice floats on lakes in the winter and why the bottom of frozen lakes tends to be about 4 °C. 00:08:13.960 --> 00:08:18.199 And also why hockey was invented. And why soda bottles explode when you leave them in the freezer. 00:08:18.200 --> 00:08:22.979 But hydrogen bonds are also why taking a warm bath is so great, why steam engines changed the world, 00:08:22.980 --> 00:08:29.020 and why temperatures on our planet are so constant when compared to other cosmic temperature fluctuations. 00:08:29.020 --> 00:08:32.429 It takes a lot of energy to change the temperature of water 00:08:32.429 --> 00:08:37.980 because each little temperature change is associated with breaking or forming lots of hydrogen bonds, 00:08:37.980 --> 00:08:40.599 and they absorb or give off a lot of heat. 00:08:40.599 --> 00:08:44.800 In fact, the specific heat capacity of water is about five times that of common rocks. 00:08:44.800 --> 00:08:49.060 And amazingly, we haven't even finished talking about how powerfully useful these partial charges are. 00:08:49.060 --> 00:08:54.120 They also allow water to dissolve pretty much anything that's even partially non-polar, 00:08:54.120 --> 00:08:58.570 which includes sugars, proteins, ions, and tons of inorganic chemicals. 00:08:58.570 --> 00:09:05.000 Water and its useful little dipole moment can dissolve more compounds than any other chemical on Earth. 00:09:05.000 --> 00:09:08.100 Frankly, it's amazing that it doesn't dissolve us from the inside out. 00:09:08.100 --> 00:09:11.830 Which brings me to one last little polarity tidbit, the hybrid molecule. 00:09:11.830 --> 00:09:15.470 There are lots of different molecules, like the surfactants in soap, for example, that 00:09:15.470 --> 00:09:17.160 have both polar and non-polar areas. 00:09:17.160 --> 00:09:25.320 Dish soap is thus able to dissolve the fatty parts of my butter catastrophe here, and then stick the polar sides out, 00:09:25.320 --> 00:09:29.840 allowing the whole mess to get washed away by Avogadro's numbers of polar water molecules 00:09:29.850 --> 00:09:31.980 that I'm sticking on my hand right now. 00:09:31.980 --> 00:09:35.230 Oh yeah. That's better, but not... 00:09:35.230 --> 00:09:40.360 I'm gonna have to go to the bathroom to get this all the way fixed up. So, be right back. 00:09:40.360 --> 00:09:44.110 Likewise, the fatty acids that make up your cell membranes have polar heads, 00:09:44.110 --> 00:09:47.240 which keeps them interacting with the aqueous environment of out bodies, 00:09:47.240 --> 00:09:52.500 but non-polar tails, which prevent the cells from being just dissolved by the water around them. 00:09:52.500 --> 00:09:54.240 Pretty dang elegant if you ask me. 00:09:54.240 --> 00:09:56.260 Thanks for watching this episode of Crash Course Chemistry. 00:09:56.260 --> 00:09:58.140 If you were paying attention, you learned that 00:09:58.140 --> 00:10:03.130 a molecule needs to have both charge asymmetry and geometric asymmetry to be non-polar, 00:10:03.130 --> 00:10:08.440 that charge asymmetry is caused by a difference in electronegativities, and that I am totally team polar. 00:10:08.440 --> 00:10:13.240 You also learned how to notate a dipole moment or charge separation of a molecule, 00:10:13.240 --> 00:10:16.430 the actual physical mechanism behind "like dissolves like", 00:10:16.430 --> 00:10:20.790 and why water is just so dang good at fostering life on this planet. 00:10:20.790 --> 00:10:23.930 This episode was written by me, edited by Blake de Pastino. 00:10:23.930 --> 00:10:27.200 Our chemistry consultants are Dr. Heiko Langner and Edi Gonzalez. 00:10:27.200 --> 00:10:30.260 It was filmed, edited, and directed by Nicholas Jenkins. 00:10:30.260 --> 00:10:35.220 Michael Aranda is our script supervisor and sound designer, and our graphics team is Thought Café.