0:00:00.280,0:00:04.500
Molecules! So many of them in their infinite[br]and beautiful variety,

0:00:04.500,0:00:08.109
but while that variety is great, it can also[br]be pretty dang overwhelming.

0:00:08.109,0:00:14.989
And so, in order to help this complicated chemical world make a little more sense, we classify and we categorize.

0:00:14.989,0:00:17.520
It's our nature as humans, and it's extremely[br]useful.

0:00:17.520,0:00:21.990
One of the most important of those classifications[br]is whether a molecule is polar or non-polar.

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It's a kind of symmetry, not just of the molecule,[br]but of the charge.

0:00:25.250,0:00:27.710
It's pretty easy to see when you're just lookin'[br]at 'em.

0:00:27.710,0:00:33.260
You got polar and non-polar, polar, non-polar,[br]polar, non-polar.

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I'm gonna take sides right now.[br]I'm on team polar.

0:00:35.670,0:00:40.510
I think polar molecules are way more interesting,[br]despite their wonky, off-balance selves.

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Non-polar molecules are useful, and their[br]symmetry has a kind of beauty,

0:00:43.190,0:00:45.990
but polar, in my humble opinion, is where[br]it's at.

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[Theme Music]

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All right. Now here are two very different[br]types of chemicals.

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Right here I have a stick of butter, and then[br]in this bowl, that's just normal water.

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So I'm just gonna go ahead and squeeze this butter, which if you're wondering is both a terrible and wonderful feeling.

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And then I'm going to [laughs] just drop that.

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Now I'm going to attempt to wash that butter[br]off my hand.

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But that is just not hap...[br]that's just, it's not going anywhere, ever.

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Ever.[br]It's just beading up on me.

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Why? Because water is a polar molecule, and the various chemicals that make up butter are non-polar,

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and water wants nothing to do with that.

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So. What makes a molecule polar?[br]Well, two things.

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First, asymmetrical electron distribution[br]around the molecule.

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You can't have a polar molecule made up entirely[br]of the same element

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because those atoms will all have the same[br]electronegativity,

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and thus the electron distribution will be[br]completely symmetrical.

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Electronegativity is usually thought of as how much an element wants electrons around it,

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but I think it's more about how much electrons[br]want to be near that element.

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If electrons were 13-year-old girls, fluorine[br]would be Niall Horan.

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They'll do anything just to be near it.[br]Why? Some simple periodic trends.

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Electronegativity increases from left to right[br]because there are more protons in the atoms,

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and more protons means more boys in the band.

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Meanwhile, it decreases as you move from top[br]to bottom

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because as the crowd of electrons gets bigger, they start to shield each other from the effects of the protons.

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What I'm trying to say is that electrons are[br]hipsters.

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If a bunch of other electrons are into that[br]thing, they're less interested.

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Now there are a number of other factors here,

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but just like the relationship between tweens[br]and their latest boy band fixation,

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it's complicated and weird and you probably[br]don't want to think too much about it.

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But in this nice little map, you can see that[br]the trend is pretty clear.

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The upper-right is where all the superstars[br]of electro-fame are.

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Oxygen, nitrogen, fluorine, chlorine, and bromine are basically the One Direction of the periodic table.

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So for polarity to occur in a molecule, you[br]have to have two different elements at a minimum,

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and the difference between their electronegativities[br]has to be 0.5 or greater.

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If that's the case, the outer electrons spend enough extra time around the element that's more electronegative

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that chemists label the molecule polar.

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The result is a partially negative charge[br]on the more electronegative part of the molecule

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and a partially positive charge on the less[br]electronegative side.

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Now in extreme cases, like if the electronegativity[br]is greater than 1.6,

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then we end up with two ions in the same molecule.

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This isn't what we're talking about here when[br]we talk about polar molecules.

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We're talking about differences between 0.5[br]and 1.6.

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Another requirement for polarity: you gotta[br]have geometrical asymmetry.

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CO2 here has the charge asymmetry locked up,[br]but because the molecule is linear, in a straight

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line, it's a kind of symmetrical asymmetry.

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The same thing does for CH4 with its tetrahedron[br]of weakly electronegative hydrogens around

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a more strongly electronegative carbon.

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These molecules have polar bonds, but the[br]molecules themselves are not polar

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because the symmetry of the bonds cancels[br]out the asymmetry of the charges.

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In order for a molecule to be polar, there[br]has to be a dipole moment,

0:03:53.550,0:03:58.480
a separation of the charge around the molecule into a more positive area and a more negative area.

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Lots of molecules are asymmetrical in both[br]electronegativity and geometry.

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Those are our polar molecules, the asymmetrical[br]beauties of chemistry.

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Look at 'em all! They're so quirky and weird!

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We've also got a system for indicating where[br]their charges are.

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We draw an arrow with a plus sign at the tail[br]pointing toward the negative side of the molecule.

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A little lowercase delta plus (δ+) or delta[br]minus (δ–) by the individual atoms signify

0:04:22.500,0:04:25.120
a partial positive pr partial negative charge.

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Liquids made up of polar molecules are really[br]good at

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dissolving solids that are composed of polar[br]or ionic compounds.

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Ionic solids are basically just polarity taken[br]to the extreme,

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so far that instead of having a partial positive[br]and partial negative dipole moment,

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the electrons have completely transferred,[br]creating two charged ions.

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Now I assume we've all heard that like dissolves[br]like,

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so the easiest way to figure out if a liquid is polar or non-polar is just to dump it in some water.

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But the why of this phenomenon is usually[br]just totally glossed over.

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What's actually happening to those molecules?

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It seems like they're all just bigots, terrified[br]of anything a little bit different than themselves.

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But this is chemistry, so there must be some[br]fundamental reason.

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And if it's fundamental, it probably has something[br]to do with decreasing the energy of the system.

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And indeed it does.

0:05:11.100,0:05:14.000
Those partial positive and partial negative[br]charges of water?

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They're at their lowest energy state when[br]they're lining up together, positive to negative,

0:05:17.920,0:05:19.960
into a kind of liquid crystal.

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There's an arrangement there.[br]It flows, of course,

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but the oxygen sides are always doing their[br]best to orient themselves toward the hydrogen

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sides of other molecules.

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You can even see the effects of that attraction

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as the surface tension that allows me to pour more than 100 milliliters of water into a 100 mil container.

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The strength of that surface tension depends on the intermolecular forces that pull molecules of a liquid together.

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These attractive, also called cohesive, forces[br]pull the surface molecules inward.

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And what you see when you look at this pile[br]of water is the result of those cohesive forces,

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minimized surface area in the water in this[br]beaker.

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When you pit a bit of oil into that mix, the[br]water totally freaks out.

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Oils have notoriously non-polar molecules,[br]so suddenly

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there's this mass of uncharged gunk interfering with the nice, orderly arrangement of polar water molecules.

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But if you take a closer look, the processes are very similar to those between water and air.

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Water does everything it can to minimize its[br]surface area and kind of expels the oil droplets.

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Rather than the water disliking the oil, it actually just likes itself much more, so it won't mix with the oil.

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Now if you put polar stuff in, water is all[br]about that,

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and those polar water molecules just go after[br]whatever other partial charges they can find.

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Or, in the case of many ionic solids,

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the partial negative charges on the oxygen[br]side all gang up on the positive ions,

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while the partial positives on the hydrogen[br]side surround the negative ions,

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breaking the crystals apart and dissolving[br]them into freely moving ions.

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In some cases we can actually witness these[br]interactions in unexpected ways.

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Mix 50 milliliters of water with 50 mils of alcohol and what the heck? There's less than 100 mils of liquid!

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The arrangement of water mixed with alcohol is actually more structured, and thus more dense,

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resulting in a smaller volume.

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The polarity of water also results in a phenomenon[br]that makes life possible: hydrogen bonding.

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The partially negative oxygen and positive[br]hydrogen atoms in a water molecule are not

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100% faithful to each other.

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They engage in additional kind of loose relationships with other neighboring hydrogen and oxygen atoms.

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These loose, somewhat fleeting relationships[br]are called hydrogen bonds.

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In ice, 100% of O and H atoms are involved[br]in hydrogen bonding.

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The most energetically favorable spatial arrangement[br]of these bonds

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actually pushes the water molecules apart[br]a bit,

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resulting in the volume of ice being 10% larger[br]than the volume of water,

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which is really weird for solids and liquids.

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When ice melts, there are still about 80%[br]of Os and Hs engaged in hydrogen bonding,

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creating ice-like clusters that keep the volume[br]of the cold water relatively high.

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With rising temperatures, these clusters disappear,

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while the volume of the truly liquid water rises resulting in a major characteristic of water:

0:08:04.700,0:08:07.759
having its highest density at 4 °C.

0:08:07.760,0:08:13.960
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.

0:08:13.960,0:08:18.199
And also why hockey was invented. And why soda bottles explode when you leave them in the freezer.

0:08:18.200,0:08:22.979
But hydrogen bonds are also why taking a warm bath is so great, why steam engines changed the world,

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and why temperatures on our planet are so constant when compared to other cosmic temperature fluctuations.

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It takes a lot of energy to change the temperature[br]of water

0:08:32.429,0:08:37.980
because each little temperature change is associated with breaking or forming lots of hydrogen bonds,

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and they absorb or give off a lot of heat.

0:08:40.599,0:08:44.800
In fact, the specific heat capacity of water[br]is about five times that of common rocks.

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And amazingly, we haven't even finished talking about how powerfully useful these partial charges are.

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They also allow water to dissolve pretty much[br]anything that's even partially non-polar,

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which includes sugars, proteins, ions, and[br]tons of inorganic chemicals.

0:08:58.570,0:09:05.000
Water and its useful little dipole moment can dissolve more compounds than any other chemical on Earth.

0:09:05.000,0:09:08.100
Frankly, it's amazing that it doesn't dissolve[br]us from the inside out.

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Which brings me to one last little polarity[br]tidbit, the hybrid molecule.

0:09:11.830,0:09:15.470
There are lots of different molecules, like[br]the surfactants in soap, for example, that

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have both polar and non-polar areas.

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Dish soap is thus able to dissolve the fatty parts of my butter catastrophe here, and then stick the polar sides out,

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allowing the whole mess to get washed away[br]by Avogadro's numbers of polar water molecules

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that I'm sticking on my hand right now.

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Oh yeah.[br]That's better, but not...

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I'm gonna have to go to the bathroom to get[br]this all the way fixed up. So, be right back.

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Likewise, the fatty acids that make up your[br]cell membranes have polar heads,

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which keeps them interacting with the aqueous[br]environment of out bodies,

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but non-polar tails, which prevent the cells from being just dissolved by the water around them.

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Pretty dang elegant if you ask me.

0:09:54.240,0:09:56.260
Thanks for watching this episode of Crash[br]Course Chemistry.

0:09:56.260,0:09:58.140
If you were paying attention, you learned[br]that

0:09:58.140,0:10:03.130
a molecule needs to have both charge asymmetry[br]and geometric asymmetry to be non-polar,

0:10:03.130,0:10:08.440
that charge asymmetry is caused by a difference in electronegativities, and that I am totally team polar.

0:10:08.440,0:10:13.240
You also learned how to notate a dipole moment[br]or charge separation of a molecule,

0:10:13.240,0:10:16.430
the actual physical mechanism behind "like[br]dissolves like",

0:10:16.430,0:10:20.790
and why water is just so dang good at fostering[br]life on this planet.

0:10:20.790,0:10:23.930
This episode was written by me, edited by[br]Blake de Pastino.

0:10:23.930,0:10:27.200
Our chemistry consultants are Dr. Heiko Langner[br]and Edi Gonzalez.

0:10:27.200,0:10:30.260
It was filmed, edited, and directed by Nicholas[br]Jenkins.

0:10:30.260,0:10:35.220
Michael Aranda is our script supervisor and sound designer, and our graphics team is Thought Café.