分子啊！ 它们有无数美丽的变种。 Molecules! So many of them and their infinite and beautiful variety,
虽然多样化是件好事，但是也多得有点恐怖。 but while that variety is great, it can also be pretty dang overwhelming.
所以，为了更好地理解这复杂的化学世界，人们分类，再分类。 And so, in order to help this complicated chemical world make a little more sense, we classify and we categorize.
这是我们人类的本性，而且它非常有用。 It's our nature as humans, and it's extremely useful.
其中，按一个分子是否为极性分子的分类方法是极为重要的。 One of the most important of those classifications is whether a molecule is polar or non-polar.
它们其实有些“对称”，不只是在分子这个层次，而是在它们带的电子数。 It's a kind of symmetry, not just of the molecule, but of the charge.
如果拿一个原子来打比方就很容易理解了。It's pretty easy to see when you just look at an atom.
这是极性分子，和非极性分子。You got polar [on screen: H2O] and non-polar [Cl2],
极性的，非极性的 polar [HCl], non-polar [CH4],
极性的，非极性的 polar [a polar bear], non-polar [a creature with two polar bear heads opposite each other.]
我现在要选一边。 I'm gonna take sides right now.
我选极性队。 I'm on team polar.
我认为极性分子虽然易动，但它们更有趣。 I think polar molecules are way more interesting, despite their wonky, off-balance selves.
非极性分子也很有用，他们的平衡性是极其漂亮的。 Non-polar molecules are useful, and their symmetry has a kind of beauty,
但是我还是更喜欢极性分子。 but polar in my humble opinion is where it's at.
好了，现在这里有两种非常不同的物质。 All right. Now here are two very different types of chemicals.
这是黄油, 碗里的是普通的水. Right here I have a stick of butter, and then in this bowl, that's just normal water.
现在我要捏下这堆黄油，如果你好奇的话，我告诉你这是可怕又奇妙的感觉。 So I'm just gonna go ahead and squeeze this butter, which if you're wondering is both a terrible and wonderful feeling.
然后我把它放在一边 And then I'm going to [laughs] just drop that.
现在我要试图把剩下的黄油从手上洗掉。 Now I'm going to attempt to wash that butter off my hand.
但那没用，黄油还在我手上， But that is just not hap. that's just, it's not going anywhere, ever. Ever.
就是黏在我手上。 It's just beading up on me.
为什么？因为水是极性分子，而组成黄油的那几种物质都是非极性的， Why? Because water is a polar molecule, and the various chemicals that make up butter are non-polar,
而水一点也不想和它们交朋友， and water wants nothing to do with that. *groans*
那么，是什么使得分子变成极性的？ So. What makes a molecule polar?
好吧，两件事，首先，是在分子周围不均匀分布的电子。 Well, two things. First, asymmetrical electron distribution around the molecule.
同一种元素是没法组成极性分子的， You can't have a polar molecule made up entirely of the same element
因为这些原子都有着同样的电负性。 because those atoms will all have the same electronegativity,
因此，在同一元素组成的原子中，电子的分布是完全对称的。 and thus the electron distribution will be completely symmetrical.
电负性经常被理解为元素想要电子在其周围的程度。 Electronegativity is usually thought of as how much an element wants electrons around it,
但我觉得电负性的定义应为电子想接近那个元素的程度。but I think it's more about how much electrons want to be near that element.
打个比方，如果把电子看作13岁的花季少女， If electrons were 13-year-old girls,
氟就是尼尔·霍拉尼，她们会不惜一切试着接近他。 fluorine would be Niall Horan. They'll do anything just to be near it.
为什么？ 几个简单的周期趋势 Why? Some simple periodic trends.
在元素周期表中，同一周期电负性从左到右增加, Electronegativity increases from left to right
因为右侧原子的质子更多， because there are more protons in the atoms,
质子更多意味着那乐队里的男生越多。 and more protons means more boys in the band.
与此同时，电负性会从上到下递减， Meanwhile, it decreases as you move from top to bottom
因为电子越来越多了， because as the crowd of electrons gets bigger,
它们会互相排挤， they start to shield each other from the effects of the protons.
我想说的就是，电子就像那些特赶时髦的人，如果太多人喜欢同一件东西，它们对其的热度就下降了不少。What I'm trying to say is that electrons are hipsters. If a bunch of other electrons are into that thing, they're less interested.
还有一些其他因素，但 Now there are a number of other factors here, but just like the relationship between tweens and their latest boy band fixation,
它复杂又奇怪，也许你不会喜欢去多想它的. it's complicated and weird and you probably don't want to think too much about it.
但在这个小地图里，你可以看到趋势挺明显的。 But in this nice little map, you can see that the trend is pretty clear.
右上方就是超级明星云集的地方。The upper-right is where all the superstars of electro-fame are.
氧，氟，氯，和溴。 Oxygen, nitrogen, fluorine, chlorine, and bromine
那几本上就是元素周期表的One Direction。 are basically the One Direction of the periodic table.
一个分子为了有极性，分子内至少要有2种不同元素。 So for polarity to occur in a molecule, you have to have two different elements at a minimum,
而且它们电负性的差得是 0.5 或者更大。 and the difference between their electronegativities has to be 0.5 or greater.
在这种情况下，最外层的电子可以在电负性更高元素外围花足够的时间。 If that's the case, the outer electrons spend enough extra time around the element
化学家就会将那列为极性原子。 that's more electronegative that chemists label the molecule polar.
结果是在原子电负性更强的一边形成一个部分为负的电极， The result is a partially negative charge on the more electronegative part of the molecule
然后在电负性较弱的一边形成一个部分为正的电极and a partially positive charge on the less electronegative side.
在特殊情况下，比如说一个分子的电负性的差大于1.6， Now in extreme cases, like if the electronegativity is greater than 1.6,
那么同一个分子里就会有2个离子。 then we end up with two ions in the same molecule.
但是那另当别论。 This isn't what we're talking about here when we talk about polar molecules.
我们现在在讲的的电负差在0.5和1.6直接的情况。 We're talking about differences between 0.5 and 1.6.
极性的另一个要求： Another requirement for polarity:
必须是不对称的，二氧化碳的电极虽然是不对称的， you gotta have geometrical asymmetry. CO2 here has the charge asymmetry locked up,
但是分子却是直线型的，是一种对称的不对称（玛德这里我真的不会翻。。）but because the molecule is linear, in a straight line, it's a kind of symmetrical asymmetry.
CH4也有同样的问题，The same thing does for CH4 with its tetrahedron of weakly electronegative hydrogens
四个电负较弱的原子围绕着相对更强的碳原子 around a more strongly electronegative carbon.
这些分子有极性键，但分子本身不是极性的， These molecules have polar bonds, but the molecules themselves are not polar
因为键的对称和电极的不对称相互抵消 because the symmetry of the bonds cancels out the asymmetry of the charges.
为了变成极性， In order for a molecule to be polar,
这一定会有一个双极的时候。 there has to be a dipole moment,
a separation of the charge around the molecule into a more positive area and a more negative area.
很多分子在电负性和几何上都不平衡。 Lots of molecules are asymmetrical in both electronegativity and geometry.
那些就是我们的极性分子，化学中不平衡的美。 Those are our polar molecules, the asymmetrical beauties of chemistry.
看看它们，那么不同 Look at 'em all! They're so quirky and weird!
我们也有一整个系统表示它们的电极。 We've also got a system for indicating where their charges are.
我们在指向向负极的的地方画一个加号 We draw an arrow with a plus sign at the tail pointing toward the negative side of the molecule.
用δ+表示正极部分或δ–表示负极部分 A little lowercase delta plus (δ+) or delta minus (δ–) by the individual atoms signify a partial positive pr partial negative charge.
由极性分子组成的液体可以很好的溶解极性分子或离子组成的固体。 Liquids made up of polar molecules are really good at dissolving solids that are composed of polar or ionic compounds.
离子固体其实就是极端化的极性（这东西我真不知道。。没学过中文版本啊。。）Ionic solids are basically just polarity taken to the extreme, so far that instead of having a partial positive
and partial negative dipole moment, the electrons have completely transferred, creating two charged ions.
Now I assume we've all heard that like dissolves like,
其实辨别一种液体是否为极性最简单的方法就是把它倒入水中 so the easiest way to figure out if a liquid is polar or non-polar is just to dump it in some water.
但是人们总是把其中的原理轻描淡写 But the why of this phenomenon is usually just totally glossed over.
到底对那些分子做了什么？ What's actually happening to those molecules?
它们看起来像一群心胸狭窄的人，害怕与它们有一丝不一样的任何东西 It seems like they're all just bigots, terrified of anything a little bit different than themselves.
但这是化学，所以肯定有一些基础的原因。 But this is chemistry, so there must be some fundamental reason.
如果很基础，那么就有可能和整个系统能量的减少有关。 And if it's fundamental, it probably has something to do with decreasing the energy of the system.
当然有关。 \ N And indeed it does.
是水的那些正极部分和负极部分造成的？ Those partial positive and partial negative charges of water?
当把正电极和负点极排起来，成为一个液晶体的时候，就是它们能量的最低点。 They're at their lowest energy state when they're lining up together, positive to negative, into a kind of liquid crystal.
就像这样。 There's an arrangement there.
它们会动，但是氧的那一边总是会努力地 It flows, of course, but the oxygen sides are always doing their best to
随着氢气而移动 orient themselves toward the hydrogen sides of other molecules.
这就是你看到水表面张力的原理。 You can even see the effects of that attraction as the surface tension
那水面张力使我可以用100毫升的容器装大于100毫升的液体。 that allows me to pour more than 100 milliliters of water into a 100 mil container.
水面张力是由分子间吸引力的大小而决定的。 The strength of that surface tension depends on the intermolecular forces that pull molecules of a liquid together.
这吸力将表面的分子向内拉动。 These attractive, also called cohesive, forces pull the surface molecules inward.
一打水就是吸力干的好戏。 And what you see when you look at this pile of water is the result of those cohesive forces,
使这个烧杯中水的表面积最小化。 minimized surface area in the water in this beaker.
当你滴进一滴油，水的反应非常剧烈. When you pit a bit of oil into that mix, the water totally freaks out.
油具有糟糕的非极性分子，所以突然这一堆无电荷的粘性物质， Oils have notoriously non-polar molecules, so suddenly there's this mass of uncharged
和有序排列的极性水分子相遇了. gunk interfering with the nice, orderly arrangement of polar water molecules.
但如果你仔细看，它很像水和空气之间反应的过程. But if you take a closer look, the processes are very similar to those between water and air.
水会尽力减少它的表面面积，在某种程度上排斥油珠. Water does everything it can to minimize its surface area and kind of expels the oil droplets.
不是水不喜欢油，只是它更喜欢自己，所以不会与油混合. Rather than the water disliking the oil, it actually just likes itself much more, so it won't mix with the oil.
现在，如果你将极性物质放进去，就是水， Now if you put polar stuff in, water is all about that, and those polar
这些极性水分子就会去追逐任何可以找到的局部电荷. water molecules just go after whatever other partial charges they can find.
或者，在离子晶体很多的情况下，氧离子这边的部分负电荷聚集到阳离子周围， Or, in the case of many ionic solids, the partial negative charges on the oxygen side all gang up
氢离子这边的正电荷聚集到负离子周围，破坏结晶，将它们溶解为自由运动的离子. on the positive ions while the partial positives on the hydrogen side surround the negative ions, breaking the crystals apart and dissolving them into freely moving ions.
有时候我们甚至可以观察到这种相互作用带来的影响. In some cases we can actually witness these interactions in unexpected ways.
将50毫升的水和50毫升的酒混合，啊呀？混合液体的体积不足100毫升！事实上混合酒水结构更紧密，因此更紧密，体积更小. Mix 50 milliliters of water with 50 mils of alcohol and what the heck? There's less than 100 mils of liquid! The arrangement of water mixed with alcohol is actually more structured, and thus more dense, resulting in a smaller volume.
水的极性也造成的一种使生命得以存在的现象：氢键作用. The polarity of water also results in a phenomenon that makes life possible: hydrogen bonding.
水分子中的部分负氧和正氢原子并非完全忠于对方. The partially negative oxygen and positive hydrogen atoms in a water molecules are not 100% faithful to each other.
它们会和周围的氢氧原子形成额外的不牢固的联系. They engage in additional kind of loose relationships with other neighboring hydrogen and oxygen atoms.
这种不牢固的、短暂的关系被称为氢键. These loose, somewhat fleeting relationships are called hydrogen bonds.
在冰里，所有的氢和氧都参与了氢键作用. In ice, 100% of O and H atoms are involved in hydrogen bonding.
其中最活跃的化学键的空间排列使水分子的距离变大，结果就是冰的体积比水要大10%，这种情况对于固体和液体来说很奇怪。 The most energetically favorable spatial arrangement of these bonds actually pushes the water molecules apart a bit, resulting in the volume of ice being 10% larger than the volume of water, which is really weird for solids and liquids.
当冰融化时，仍有80%的氢和水参与氢键作用，形成像冰一样的小块，使冰水保持相对较大的体积。 When ice melts, there are still about 80% of Os and Hs engaged in hydrogen bonding, creating ice-like clusters that keep the volume of the cold water relatively high.
随着温度继续升高，小快消失，同时真正液态水的体积继续增加，在摄氏4度的时候达到最大，这是水的一个主要特征。 WIth rising temperatures, these clusters disappear, while the volume of the truly liquid water rises resulting in a major characteristic of water, having its highest density at 4 °C.
是的，这就是为什么冬天河里的冰会在河面上，并且结冰的河水的温度在摄氏四度左右. 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.
这也是发明冰球的原因， And also why hockey was invented.
苏打水放在冷冻室会爆炸的原因. And why soda bottles explode when you leave them in the freezer.
但是氢键作用也是热水澡如此舒服、蒸汽机得以改变世界和相比宇宙，地球的温度如此恒定的原因. But hydrogen bonds are also why taking a warm bath is so great, why steam engines changed the world, and why temperatures on our planet are so constant when compared to other cosmic temperature fluctuations.
改变水的温度需要大量能量，因为一点点温度的改变都和氢键的分裂和结合相关，这个过程会吸收和放出大量的热量. It takes a lot of energy to change the temperature of water because each little temperature change is associated with breaking or forming lots of hydrogen bonds, and they absorb or give off a lot of heat.
事实上，水的比热容是普通石头的五倍。 In fact, the specific heat capacity of water is about five times that of common rocks.
令人惊讶的是，我们还没谈完这些局部电荷是多么的有用. And amazingly, we haven't even finished talking about how useful these partial charges are.
它们使水得以溶解几乎任何东西，甚至部分非极性的物体，包括糖、蛋白质、离子和大量的无机化学材料. They also allow water to dissolve pretty much anything that's even partially non-polar, which includes sugars, proteins, ions, and tons of inorganic chemicals.
水和它有用的电偶极矩溶解化合物的能力超过地球上其他任何物质. Water and its useful little dipole moment can dissolve more compounds than any other chemical on Earth.
老实说，水没把我们自内向外溶解挺令人惊讶的. Frankly, it's amazing that it doesn't dissolve us from the inside out.
这就要谈到今天最后一个极性趣闻：杂交分子. Which brings me to one last little polarity tidbit, the hybrid molecule.
很多不同的分子，比如肥皂里的表面活性剂，同时拥有极性和非极性区域. There are lots of different molecules, like the surfactants in soap, for example, that have both polar and non-polar areas.
因此，洗碗皂可以溶解我这儿黄油的脂肪部分，把极性部分剔除，再让大量的极性水分子把粘在我手上的东西洗掉. Dish soap is thus able to dissolve the fatty parts of my butter catastrophe here, and then stick the polar sides out, allowing the whole mess to get washed away by Avogadro's numbers of polar water molecules that I'm sticking on my hand right now.
哦耶. Oh yeah.
这样好多了，但还不够。 That's better, but not.
我要去洗手间把这些东西全部处理了. I'm gonna have to go to the bathroom to get this all the way fixed up.
马上回来. So, be right back.
同样的，组成人体细胞膜的脂肪酸具有极性头，阻止它们与身体的水相环境相互作用，然而阻止细胞生成的非极性头便被水溶解了. Likewise, the fatty acids that make up your cell membranes have polar heads, which keeps them interacting with the aqueous environment of out bodies, but non-polar tails, which prevent the cells from being just dissolved by the water around them.
依我看来非常优雅. Pretty dang elegant if you ask me.
感谢收看本期化学速成. Thanks for watching this episode of Crash Course Chemistry.
如果你认真看了，你会学到分子需要同时做到电荷对称和几何对称才能变成非极性，电荷对称是电负性的不同造成的. If you were paying attention, you learned that a molecule needs to have both charge asymmetry and geometric asymmetry to be non-polar, that charge asymmetry is caused by a difference in electronegativities, and that I am totally team polar.
同时你会学到如何用符号表示偶极矩和分子的电荷分离，相似相溶原理背后的物理机制和为什么水如此适合养育地球上的生命. You also learned how to notate a dipole moment or charge separation of a molecule, the actual physical mechanism behind "like dissolves like", and why water is just so dang good at fostering life on this planet.