You wish you had something ice-cold to dull the pain,

你會想用冰敷減緩疼痛，

but to use an ice pack, you would have had to put it in the freezer hours ago.

但使用冰袋，你得在數小時前 把它放入冰箱裡才行。

Fortunately, there's another option.

還好，我們還有其他的選擇，

A cold pack can be left at room temperature until the moment you need it,

冰敷袋可以在室溫下保存， 直到你需要它的那一刻，

then just snap it as instructed and within seconds you'll feel the chill.

使用幾秒鐘後就會讓你覺得冰涼，

But how can something go from room temperature to near freezing

但是，如何在這麼短的時間內

in such a short time?

由室溫降到接近冰點呢？

The answer lies in chemistry.

答案在化學裡。

Your cold pack contains water and a solid compound,

你的冰敷袋中裝有水和固體化合物，

usually ammonium nitrate, in different compartments separated by a barrier.

通常是硝酸銨，袋中物質由屏障分隔。

When the barrier is broken, the solid dissolves

當屏障被打破，固體遇水溶解，

causing what's known as an endothermic reaction,

就會發生吸熱反應，

one that absorbs heat from its surroundings.

反應時由周遭環境中吸收熱量，

To understand how this works,

要了解其工作原理，

we need to look at the two driving forces behind chemical processes:

我們需要看一下化學反應 背後的兩種驅動力：

energetics and entropy.

熱力學和熵，

These determine whether a change occurs in a system and how energy flows if it does.

這些決定反應發生與否及能量流動情形。

In chemistry, energetics deals with the attractive and repulsive forces

在化學中，熱力學處理粒子與分子層次中

between particles at the molecular level.

吸引力和排斥力問題。

This scale is so small that there are more water molecules in a single glass

在這個尺度下，一杯水所含的分子數目

than there are known stars in the universe.

比已知的宇宙星星還多。

And all of these trillions of molecules are

這些萬億個分子

constantly moving, vibrating and rotating at different rates.

以不同的速率不斷移動、振動和旋轉。

We can think of temperature as a measurement of the average motion,

我們可將溫度視為這些 運動平均的度量結果。

or kinetic energy, of all these particles,

或者也可代表粒子的平均動能。

with an increase in movement meaning an increase in temperature,

粒子運動愈激烈、溫度就越高。

and vice versa.

反之亦然。

The flow of heat in any chemical transformation

熱在任何化學變化中的轉換，

depends on the relative strength of particle interactions

取決於在每一種物質在特定化學狀態時，

in each of a substance's chemical states.

粒子相互作用的相對強度。

When particles have a strong mutual attractive force,

當粒子具有很強的吸引力時，

they move rapidly towards one another, until they get so close,

它們彼此快速靠近到很近的距離，

that repulsive forces push them away.

造成排斥力作用，推開彼此。

If the initial attraction was strong enough,

如果最初的吸引力夠強大，

the particles will keep vibrating back and forth in this way.

粒子將以此方式來回震盪。

The stronger the attraction, the faster their movement,

吸引力越強大、運動越快，

and since heat is essentially motion,

因為熱的本質就是運動，

when a substance changes to a state in which these interactions are stronger,

當物質狀態改變時，交互作用變得更強，

the system heats up.

系統即增溫。

But our cold packs do the opposite,

但冰敷袋的情形恰巧相反，

which means that when the solid dissolves in the water,

當固體溶解在水中時，

the new interactions of solid particles and water molecules with each other

固體粒子與水分子的交互作用

are weaker than the separate interactions that existed before.

比之前個別單獨存在時弱，

This makes both types of particles slow down on average,

所以平均來看，兩種粒子都變慢了，

cooling the whole solution.

所以溶液整體是變冷的。

But why would a substance change to a state where the interactions were weaker?

但為何物質狀態改變時， 交互作用會變弱？

Wouldn't the stronger preexisting interactions keep the solid from dissolving?

先前存在較強的交互作用力， 是否會使固體無法溶解呢？

This is where entropy comes in.

這就要用熵來解釋了。

Entropy basically describes how objects and energy

熵基本上是描述物體與能量

are distributed based on random motion.

隨機動態分布的程度。

If you think of the air in a room, there are many different possible arrangements

如果你考慮房間裡的空氣，

for the trillions of particles that compose it.

億萬個分子會有許多 可能的相異位置分布情形，

Some of these will have all the oxygen molecules in one area,

其中可能一個區域中都是氧分子，

and all the nitrogen molecules in another.

另一個區域中都是氮分子，

But far more will have them mixed together,

但更可能是兩者混在一起，

which is why air is always found in this state.

這就是為什麼找到的空氣總是這種狀態。

Now, if there are strong attractive forces between particles,

現在，如果兩種氣體分子間 有很強的吸引力，

the probability of some configurations can change

氣體分子分布的情形就會改變了，

even to the point where the odds don't favor certain substances mixing.

甚至有某物不喜歡 與他種物質相混合的地步。

Oil and water not mixing is an example.

油和水不相互混合就是一個例子。

But in the case of the ammonium nitrate, or other substance in your cold pack,

但硝酸銨或者冰敷袋內容物的情形，

the attractive forces are not strong enough to change the odds,

吸引力未強大到不溶於水，

and random motion makes the particles composing the solid separate

再者，隨機運動使得顆粒組成的固體

by dissolving into the water and never returning to their solid state.

因溶解到水中而分離， 且再也無法回復到固體的狀態。

To put it simply, your cold pack gets cold because random motion

簡單來說，你的冰敷袋會變冷，

creates more configurations where the solid and water mix together

源自於固體和水混合時， 隨機運動使粒子分布情形變得雜亂，

and all of these have even weaker particle interaction,

這些都讓粒子交互作用變得更弱，

less overall particle movement,

整體粒子的運動更少了，

and less heat than there was inside the unused pack.

具有比未混和前較少的熱能。

So while the disorder that can result from entropy

因此來自熵的混亂，

may have caused your injury in the first place,

也許先前造成你的傷害，

its also responsible for that comforting cold that soothes your pain.

但也很盡責的藉低溫減緩你的疼痛。