So you just strained a muscle and the inflammation is unbearable.
你的肌肉刚刚拉伤了，疼痛和灼烧感让你坐卧难安。
You wish you had something ice-cold to dull the pain,
你希望能有些冰的东西来镇痛，
but to use an ice pack, you would have had to put it in the freezer hours ago.
但要是用冰袋，你需要提早几小时把它放到冷冻室里。
Fortunately, there's another option.
幸运的是，还有另一个选择。
A cold pack can be left at room temperature until the moment you need it,
一个速冻袋可以一直放置在室温下，在你需要它的时候，
then just snap it as instructed and within seconds you'll feel the chill.
只需按照指示摁下它，然后不出几秒你就会感到寒意。
But how can something go from room temperature to near freezing in such a short time?
但是某样东西是如何在这么短的时间内从室温降到接近冰点的呢？
The answer lies in chemistry.
答案就藏在化学反应中。
Your cold pack contains water and a solid compound,
你的速冻袋含有水和一种固体化合物，
usually ammonium nitrate, in different compartments separated by a barrier.
通常是硝酸铵，被间隔物阻挡在另一个区域。
When the barrier is broken, the solid dissolves causing what's known as an endothermic reaction,
当间隔物被打破，固体就溶化了，并导致了众所周知的吸热反应，
one that absorbs heat from its surroundings.
从周围的环境中吸收热量。
To understand how this works, we need to look at the two driving forces behind chemical processes: energetics and entropy.
为了理解其中的工作原理，我们需要通过化学过程了解其背后的两种动力：热力学和熵。
These determine whether a change occurs in a system and how energy flows if it does.
这些决定了系统中是否会发生变化，以及如果发生了变化能量如何流动。
In chemistry, energetics deals with the attractive and repulsive forces between particles at the molecular level.
在化学中，热力学影响了分子层面粒子间的引力和斥力。
This scale is so small that there are more water molecules in a single glass than there are known stars in the universe.
分子是如此的小，以至于一杯水中的水分子比宇宙中已知的星星数量还要多。
And all of these trillions of molecules are constantly moving, vibrating and rotating at different rates.
所有这些数万亿的分子一直以不同的速度保持运动，振颤和旋转。
We can think of temperature as a measurement of the average motion, or kinetic energy, of all these particles,
我们可以将温度看作平均运动下的标尺，或者这些所有粒子的动能，
with an increase in movement meaning an increase in temperature, and vice versa.
速度的提升意味着温度的提升，反之亦然。
The flow of heat in any chemical transformation
在任何化学变化中热量的流动，
depends on the relative strength of particle interactions in each of a substance's chemical states.
都取决于一个物质各个化学状态下其粒子间互相作用的相对强弱。
When particles have a strong mutual attractive force,
当粒子间有很强的相互引力，
they move rapidly towards one another, until they get so close, that repulsive forces push them away.
它们会急速地向另一个运动，直到近到会被斥力推开。

If the initial attraction was strong enough, the particles will keep vibrating back and forth in this way.
如果初始吸引力足够强大，粒子会像这样保持前后震动。
The stronger the attraction, the faster their movement, and since heat is essentially motion,
引力越大，运动越快，因为热量本质上是一种运动，
when a substance changes to a state in which these interactions are stronger, the system heats up.
当一种物质进入相互作用更强的状态时，系统就会升温。
But our cold packs do the opposite, which means that when the solid dissolves in the water,
但是速冻袋与之相反，这意味着当固体溶解在水中，
the new interactions of solid particles and water molecules with each other are weaker than the separate interactions that existed before.
固体粒子和水分子之间的新的引力会比之前分别存在的单独相互作用弱。
This makes both types of particles slow down on average, cooling the whole solution.
这会使两种类型的粒子平均上减速，使溶液降温。
But why would a substance change to a state where the interactions were weaker?
但是物质是怎么达到相互作用更弱的状态的呢？
Wouldn't the stronger preexisting interactions keep the solid from dissolving?
难道之前更强的相互作用不会阻止固体溶解吗？
This is where entropy comes in.
这就是熵参与的地方。
Entropy basically describes how objects and energy are distributed based on random motion.
熵基本上描述了物质和能量是基于随机运动分布的。
If you think of the air in a room, there are many different possible arrangements for the trillions of particles that compose it.
考虑一下房间里的空气，组成它的万亿粒子有很多种不同的排列。
Some of these will have all the oxygen molecules in one area, and all the nitrogen molecules in another.
某个区域会聚集所有的氧分子，在另一区域则聚集了所有氮分子。
But far more will have them mixed together, which is why air is always found in this state.
它们当中绝大多数会混合起来，这就是为什么空气总在这种状态下被发现。
Now, if there are strong attractive forces between particles,
现在，如果粒子间有更强的引力，
the probability of some configurations can change even to the point where the odds don't favor certain substances mixing.
一些构造产生的可能性就会发生改变，甚至会达到使某些物质不相容的状态。
Oil and water not mixing is an example.
油和水不能混合就是个例子。
But in the case of the ammonium nitrate, or other substance in your cold pack,
但是对于硝酸铵或者速冻袋中其他的物质，
the attractive forces are not strong enough to change the odds,
引力还不足以强到改变相溶性，
and random motion makes the particles composing the solid separate by dissolving into the water and never returning to their solid state.
随机运动会让构成固体的粒子通过溶解到水中而分解，并且不能恢复到固体状态。
To put it simply, your cold pack gets cold because random motion creates more configurations where the solid and water mix together
简单来讲，你的速冻袋变冷是因为随机运动创造出了固体和水混合的状态，
and all of these have even weaker particle interaction, less overall particle movement,
这些粒子间的引力甚至会更弱，整个的粒子运动越少，
and less heat than there was inside the unused pack.
热量就会比未使用过的速冻袋要少。
So while the disorder that can result from entropy may have caused your injury in the first place,
所以熵造成的紊乱也许不但导致了你的伤势，
its also responsible for that comforting cold that soothes your pain.
也造成了能舒缓你疼痛的冰爽感。