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Why does salt dissolve in water but oil doesn't?
为什么盐可溶于水而油却不行呢？
Well, in a word,chemistry. But that's not very satisfied, isn't it?
总的来说，是化学作用。但这并不令人满意，对吧？
Well, the reason salt dissolves and oil does not comes down to the two big reasons why anything happens at all: energetics and entropy.
其实，盐可溶于水而油不能的原因可归结为一切现象发生的两个根本原因：力能学和熵。
Energetics deals primarily with the attractive forces between things.
力能学主要讨论物体之间的引力。
When we look at oil or salt in water, we focus on the forces between particles on a very, very, very small scale, the molecular level.
我们观察水中的盐或油时，会在一个非常微小的尺度即分子层级内关注粒子之间的作用力。
To give you a sense of this scale, in one glass of water, there are more molecules than known stars in the universe.
这个尺度究竟有多微小呢？在一杯水里，分子的数量比宇宙中已知的星星数量还要大。
Now, all of these molecules are in constant motion, moving, vibrating, and rotating.
现在，所有这些分子都在做匀速运动，移动、振动并且旋转。
What prevents almost all of those molecules from just flying out of the glass are the attractive interactions between molecules.
阻止几乎所有分子从杯子里飞出去的是分子之间相互吸引的作用力。
The strength of the interactions between water, itself, and other substances is what we mean when we say energetics.
水之间以及它与其他物质之间相互作用的力就是我们所说的力能学。
You can think of the water molecules engaging in a constant dance, sort of like a square dance where they constantly and randomly exchange partners.
你可以想象水分子始终如一地舞蹈着，它有点儿像那种经常且随机更换舞伴的方块舞。
Put simply, the ability for substances to interact with water, balanced with how they disrupt how water interacts with itself, plays an important role in explaining why certain things mix well into water and others don't.
简言之，物质与水相互作用的能力，那种平衡水与自身的相互作用能力，在解释有些物质可溶于水而另一些却不能时扮演着重要角色。
Entropy basically describes the way things and energy can be arranged based on random motion.
熵基本描述了无规则运动基础上物质与能量的组合方式。
For example, think of the air in a room.
例如，想象一个房间里的空气。
Imagine all the different possible arrangements in space for the trillions of particles that make up the air.
想象一个空间里数万亿个空气分子所有可能的组合方式。
Some of those arrangements might have all the oxygen molecules over here and all the nitrogen molecules over there, separated.
其中一些组合方式可能会将所有的氧分子置放在一边，而将所有氮分子放在那一边，使二者分离开来。
But far more of the possible arrangements have those molecules mixed up with one another.
然而，更多可能的方式是使这些分子彼此混合起来。
So, entropy favors mixing.
因此，熵倾向于讨论“混合”。
Energetics deals with attractive forces.
力能学则讨论引力。
And so, if attractive forces are present, the probability of some arrangements can be enhanced, the ones where things are attracted to each other.
故而，如果引力存在，一部分组合方式的可能性会被提高，也就是那些物体彼此吸引的方式。
So, it is always the balance of these two things that determines what happens.
所以，决定发生现象的通常是这二者的平衡。
On the molecular level, water is comprised of water molecules, made up of two hydrogen atoms and an oxygen atom.
在分子层级上，水是由水分子组成的，水分子由两个氢原子和一个氧原子构成。
