1 00:00:00,280 --> 00:00:04,500 Molecules! So many of them in their infinite and beautiful variety, 2 00:00:04,500 --> 00:00:08,109 but while that variety is great, it can also be pretty dang overwhelming. 3 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. 4 00:00:14,989 --> 00:00:17,520 It's our nature as humans, and it's extremely useful. 5 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. 6 00:00:21,990 --> 00:00:25,250 It's a kind of symmetry, not just of the molecule, but of the charge. 7 00:00:25,250 --> 00:00:27,710 It's pretty easy to see when you're just lookin' at 'em. 8 00:00:27,710 --> 00:00:33,260 You got polar and non-polar, polar, non-polar, polar, non-polar. 9 00:00:33,260 --> 00:00:35,670 I'm gonna take sides right now. I'm on team polar. 10 00:00:35,670 --> 00:00:40,510 I think polar molecules are way more interesting, despite their wonky, off-balance selves. 11 00:00:40,510 --> 00:00:43,190 Non-polar molecules are useful, and their symmetry has a kind of beauty, 12 00:00:43,190 --> 00:00:45,990 but polar, in my humble opinion, is where it's at. 13 00:00:45,990 --> 00:00:55,160 [Theme Music] 14 00:00:55,160 --> 00:00:58,479 All right. Now here are two very different types of chemicals. 15 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. 16 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. 17 00:01:08,660 --> 00:01:13,140 And then I'm going to [laughs] just drop that. 18 00:01:13,140 --> 00:01:16,620 Now I'm going to attempt to wash that butter off my hand. 19 00:01:16,620 --> 00:01:21,200 But that is just not hap... that's just, it's not going anywhere, ever. 20 00:01:21,200 --> 00:01:23,900 Ever. It's just beading up on me. 21 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, 22 00:01:29,740 --> 00:01:33,440 and water wants nothing to do with that. 23 00:01:33,440 --> 00:01:37,450 So. What makes a molecule polar? Well, two things. 24 00:01:37,450 --> 00:01:40,710 First, asymmetrical electron distribution around the molecule. 25 00:01:40,710 --> 00:01:44,030 You can't have a polar molecule made up entirely of the same element 26 00:01:44,030 --> 00:01:46,970 because those atoms will all have the same electronegativity, 27 00:01:46,970 --> 00:01:50,650 and thus the electron distribution will be completely symmetrical. 28 00:01:50,650 --> 00:01:53,910 Electronegativity is usually thought of as how much an element wants electrons around it, 29 00:01:53,910 --> 00:01:57,810 but I think it's more about how much electrons want to be near that element. 30 00:01:57,810 --> 00:02:02,060 If electrons were 13-year-old girls, fluorine would be Niall Horan. 31 00:02:02,060 --> 00:02:07,670 They'll do anything just to be near it. Why? Some simple periodic trends. 32 00:02:07,670 --> 00:02:11,940 Electronegativity increases from left to right because there are more protons in the atoms, 33 00:02:11,940 --> 00:02:14,990 and more protons means more boys in the band. 34 00:02:14,990 --> 00:02:18,050 Meanwhile, it decreases as you move from top to bottom 35 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. 36 00:02:23,180 --> 00:02:25,720 What I'm trying to say is that electrons are hipsters. 37 00:02:25,730 --> 00:02:29,130 If a bunch of other electrons are into that thing, they're less interested. 38 00:02:29,130 --> 00:02:30,850 Now there are a number of other factors here, 39 00:02:30,850 --> 00:02:34,040 but just like the relationship between tweens and their latest boy band fixation, 40 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. 41 00:02:37,050 --> 00:02:39,380 But in this nice little map, you can see that the trend is pretty clear. 42 00:02:39,380 --> 00:02:42,740 The upper-right is where all the superstars of electro-fame are. 43 00:02:42,740 --> 00:02:47,240 Oxygen, nitrogen, fluorine, chlorine, and bromine are basically the One Direction of the periodic table. 44 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, 45 00:02:51,720 --> 00:02:55,700 and the difference between their electronegativities has to be 0,5 or greater. 46 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 47 00:03:00,460 --> 00:03:02,860 that chemists label the molecule polar. 48 00:03:02,860 --> 00:03:06,400 The result is a partially negative charge on the more electronegative part of the molecule 49 00:03:06,400 --> 00:03:09,540 and a partially positive charge on the less electronegative side. 50 00:03:09,540 --> 00:03:12,599 Now in extreme cases, like if the electronegativity is greater than 1,6, 51 00:03:12,599 --> 00:03:16,020 then we end up with two ions in the same molecule. 52 00:03:16,020 --> 00:03:19,160 This isn't what we're talking about here when we talk about polar molecules. 53 00:03:19,160 --> 00:03:22,410 We're talking about differences between 0,5 and 1,6. 54 00:03:22,410 --> 00:03:26,130 Another requirement for polarity: you gotta have geometrical asymmetry. 55 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 56 00:03:31,430 --> 00:03:34,209 line, it's a kind of symmetrical asymmetry. 57 00:03:34,209 --> 00:03:39,209 The same thing does for CH4 with its tetrahedron of weakly electronegative hydrogens around 58 00:03:39,209 --> 00:03:41,180 a more strongly electronegative carbon. 59 00:03:41,180 --> 00:03:45,380 These molecules have polar bonds, but the molecules themselves are not polar 60 00:03:45,380 --> 00:03:49,319 because the symmetry of the bonds cancels out the asymmetry of the charges. 61 00:03:49,319 --> 00:03:53,550 In order for a molecule to be polar, there has to be a dipole moment, 62 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. 63 00:03:58,480 --> 00:04:02,560 Lots of molecules are asymmetrical in both electronegativity and geometry. 64 00:04:02,569 --> 00:04:06,760 Those are our polar molecules, the asymmetrical beauties of chemistry. 65 00:04:06,760 --> 00:04:09,140 Look at 'em all! They're so quirky and weird! 66 00:04:09,140 --> 00:04:12,879 We've also got a system for indicating where their charges are. 67 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. 68 00:04:17,940 --> 00:04:22,500 A little lowercase delta plus (δ+) or delta minus (δ–) by the individual atoms signify 69 00:04:22,500 --> 00:04:25,120 a partial positive pr partial negative charge. 70 00:04:25,120 --> 00:04:27,530 Liquids made up of polar molecules are really good at 71 00:04:27,530 --> 00:04:31,710 dissolving solids that are composed of polar or ionic compounds. 72 00:04:31,710 --> 00:04:34,340 Ionic solids are basically just polarity taken to the extreme, 73 00:04:34,340 --> 00:04:38,060 so far that instead of having a partial positive and partial negative dipole moment, 74 00:04:38,060 --> 00:04:41,389 the electrons have completely transferred, creating two charged ions. 75 00:04:41,389 --> 00:04:44,180 Now I assume we've all heard that like dissolves like, 76 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. 77 00:04:48,620 --> 00:04:52,960 But the why of this phenomenon is usually just totally glossed over. 78 00:04:52,970 --> 00:04:55,590 What's actually happening to those molecules? 79 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. 80 00:05:00,870 --> 00:05:04,720 But this is chemistry, so there must be some fundamental reason. 81 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. 82 00:05:09,940 --> 00:05:11,100 And indeed it does. 83 00:05:11,100 --> 00:05:14,000 Those partial positive and partial negative charges of water? 84 00:05:14,000 --> 00:05:17,920 They're at their lowest energy state when they're lining up together, positive to negative, 85 00:05:17,920 --> 00:05:19,960 into a kind of liquid crystal. 86 00:05:19,960 --> 00:05:23,120 There's an arrangement there. It flows, of course, 87 00:05:23,120 --> 00:05:27,840 but the oxygen sides are always doing their best to orient themselves toward the hydrogen 88 00:05:27,840 --> 00:05:29,030 sides of other molecules. 89 00:05:29,030 --> 00:05:31,550 You can even see the effects of that attraction 90 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. 91 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. 92 00:05:43,920 --> 00:05:48,760 These attractive, also called cohesive, forces pull the surface molecules inward. 93 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, 94 00:05:53,860 --> 00:05:56,460 minimized surface area in the water in this beaker. 95 00:05:56,460 --> 00:06:00,229 When you pit a bit of oil into that mix, the water totally freaks out. 96 00:06:00,229 --> 00:06:03,660 Oils have notoriously non-polar molecules, so suddenly 97 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. 98 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. 99 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. 100 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. 101 00:06:26,949 --> 00:06:29,770 Now if you put polar stuff in, water is all about that, 102 00:06:29,770 --> 00:06:34,229 and those polar water molecules just go after whatever other partial charges they can find. 103 00:06:34,229 --> 00:06:36,630 Or, in the case of many ionic solids, 104 00:06:36,630 --> 00:06:41,139 the partial negative charges on the oxygen side all gang up on the positive ions, 105 00:06:41,139 --> 00:06:44,910 while the partial positives on the hydrogen side surround the negative ions, 106 00:06:44,910 --> 00:06:49,080 breaking the crystals apart and dissolving them into freely moving ions. 107 00:06:49,080 --> 00:06:53,070 In some cases we can actually witness these interactions in unexpected ways. 108 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! 109 00:06:59,960 --> 00:07:05,200 The arrangement of water mixed with alcohol is actually more structured, and thus more dense, 110 00:07:05,200 --> 00:07:06,979 resulting in a smaller volume. 111 00:07:06,979 --> 00:07:12,720 The polarity of water also results in a phenomenon that makes life possible: hydrogen bonding. 112 00:07:12,720 --> 00:07:17,160 The partially negative oxygen and positive hydrogen atoms in a water molecule are not 113 00:07:17,160 --> 00:07:19,479 100% faithful to each other. 114 00:07:19,480 --> 00:07:25,020 They engage in additional kind of loose relationships with other neighboring hydrogen and oxygen atoms. 115 00:07:25,020 --> 00:07:29,039 These loose, somewhat fleeting relationships are called hydrogen bonds. 116 00:07:29,039 --> 00:07:33,499 In ice, 100% of O and H atoms are involved in hydrogen bonding. 117 00:07:33,500 --> 00:07:37,220 The most energetically favorable spatial arrangement of these bonds 118 00:07:37,229 --> 00:07:40,759 actually pushes the water molecules apart a bit, 119 00:07:40,759 --> 00:07:45,550 resulting in the volume of ice being 10% larger than the volume of water, 120 00:07:45,550 --> 00:07:47,850 which is really weird for solids and liquids. 121 00:07:47,850 --> 00:07:52,610 When ice melts, there are still about 80% of Os and Hs engaged in hydrogen bonding, 122 00:07:52,610 --> 00:07:57,289 creating ice-like clusters that keep the volume of the cold water relatively high. 123 00:07:57,289 --> 00:07:59,220 With rising temperatures, these clusters disappear, 124 00:07:59,220 --> 00:08:04,700 while the volume of the truly liquid water rises resulting in a major characteristic of water: 125 00:08:04,700 --> 00:08:07,759 having its highest density at 4 °C. 126 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. 127 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. 128 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, 129 00:08:22,980 --> 00:08:29,020 and why temperatures on our planet are so constant when compared to other cosmic temperature fluctuations. 130 00:08:29,020 --> 00:08:32,430 It takes a lot of energy to change the temperature of water 131 00:08:32,430 --> 00:08:37,980 because each little temperature change is associated with breaking or forming lots of hydrogen bonds, 132 00:08:37,980 --> 00:08:40,599 and they absorb or give off a lot of heat. 133 00:08:40,599 --> 00:08:44,800 In fact, the specific heat capacity of water is about five times that of common rocks. 134 00:08:44,800 --> 00:08:49,060 And amazingly, we haven't even finished talking about how powerfully useful these partial charges are. 135 00:08:49,060 --> 00:08:54,120 They also allow water to dissolve pretty much anything that's even partially non-polar, 136 00:08:54,120 --> 00:08:58,570 which includes sugars, proteins, ions, and tons of inorganic chemicals. 137 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. 138 00:09:05,000 --> 00:09:08,100 Frankly, it's amazing that it doesn't dissolve us from the inside out. 139 00:09:08,100 --> 00:09:11,830 Which brings me to one last little polarity tidbit, the hybrid molecule. 140 00:09:11,830 --> 00:09:15,470 There are lots of different molecules, like the surfactants in soap, for example, that 141 00:09:15,470 --> 00:09:17,160 have both polar and non-polar areas. 142 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, 143 00:09:25,320 --> 00:09:29,840 allowing the whole mess to get washed away by Avogadro's numbers of polar water molecules 144 00:09:29,850 --> 00:09:31,980 that I'm sticking on my hand right now. 145 00:09:31,980 --> 00:09:35,230 Oh yeah. That's better, but not... 146 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. 147 00:09:40,360 --> 00:09:44,110 Likewise, the fatty acids that make up your cell membranes have polar heads, 148 00:09:44,110 --> 00:09:47,240 which keeps them interacting with the aqueous environment of out bodies, 149 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. 150 00:09:52,500 --> 00:09:54,240 Pretty dang elegant if you ask me. 151 00:09:54,240 --> 00:09:56,260 Thanks for watching this episode of Crash Course Chemistry. 152 00:09:56,260 --> 00:09:58,140 If you were paying attention, you learned that 153 00:09:58,140 --> 00:10:03,130 a molecule needs to have both charge asymmetry and geometric asymmetry to be non-polar, 154 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. 155 00:10:08,440 --> 00:10:13,240 You also learned how to notate a dipole moment or charge separation of a molecule, 156 00:10:13,240 --> 00:10:16,430 the actual physical mechanism behind "like dissolves like", 157 00:10:16,430 --> 00:10:20,790 and why water is just so dang good at fostering life on this planet. 158 00:10:20,790 --> 00:10:23,930 This episode was written by me, edited by Blake de Pastino. 159 00:10:23,930 --> 00:10:27,200 Our chemistry consultants are Dr. Heiko Langner and Edi Gonzalez. 160 00:10:27,200 --> 00:10:30,260 It was filmed, edited, and directed by Nicholas Jenkins. 161 00:10:30,260 --> 00:10:35,220 Michael Aranda is our script supervisor and sound designer, and our graphics team is Thought Café.