Under the hood: The chemistry of cars - Cynthia Chubbuck

Under the hood: The chemistry of cars - Cynthia Chubbuck

翻译人员: xuanyu shi 校对人员: Jenny Yang

There are over one billion cars in the world today,

如今世界上有十亿辆车，

getting people where they need to go,

它们可以把人们带到需要去的地方

but cars aren't just a mode of transportation,

但是车子并不只是一种交通工具

they're also a chemistry lesson waiting to be taught.

它们本身也是一门需要去学习的化学课

The process of starting your car begins in the engine cylinders,

车子的启动过程从发动机气缸开始，

where a spritz of gasoline from the fuel injector

汽油从喷油器中喷出

and a gulp of air from the intake valve

然后大量的空气进入进气阀

mix together before being ignited by a spark,

在被火花点燃之前混合在一起

forming gases that expand and push the piston.

以此形成气体,扩大和推动活塞

But combustion is an exothermic reaction, meaning it releases heat.

但燃烧是一个放热反应,这意味着释放热量。

Lots of it.

而且是很多热量

And while much of this heat escapes through the tail pipe,

然而当这些热量从排气尾管释放的时候，

the heat that remains in the engine block needs to be absorbed, transported, and dissipated

剩余在引擎的热量需要吸收、运输和释放

to protect the metal components from deforming or even melting.

来避免金属部件变形甚至融化。

This is where the cooling system comes in.

这就是冷却系统的由来。

A liquid gets circulated throughout the engine,

让一种液体在整个引擎边上流通,

but what kind of liquid can absorb all that heat?

但是哪一种液体可以吸收那样的热量呢？

Water may seem like an obvious first choice.

显然水会是第一选择。

After all, its specific heat,

毕竟，它的比热容，

the amount of energy required to raise the temperature

也就是一定质量的物质每提高一摄氏度所需的能量

of a given amount by one degree Celsius,

is higher than that of any other common substance.

比任何其他的常见物质的比热容都高

And we have a lot of heat energy to absorb.

而且我们有很多需要吸收的能量

But using water can get us into deep trouble.

但是用水来冷却会带来更大的麻烦

首先，水的冰点是零摄氏度

For one thing, its freezing point is zero degrees Celsius.

Since water expands as it freezes,

由于水在结冰的时候会扩大体积，

a cold winter night could mean a cracked radiator and a damaged engine block,

一个寒冷冬天的夜晚也就意味着

一个破碎的散热器和一个损坏的引擎

a chilling prospect.

这是一个令人心寒的前景

And considering how hot car engines can get,

考虑到引擎可以承受的热量

the relatively low boiling point of 100 degrees Celsius

相对100摄氏度较低的沸点

can lead to a situation that would get anyone steamed.

会导致一种后果，那就是人们会被蒸熟

So, instead of water, we use a solution,

所以，我们用了一种溶液取代了水

a homogeneous mixture consisting of a solute and a solvent.

一种由溶质和溶剂组成的均匀的混合物。

Some of the solution's properties will differ depending on the proportion of solute present.

其中一些溶质的特性是根据溶剂的量而改变的。

These are called colligative properties, and as luck would have it,

这些特性被称为溶液的依数性，

幸运的是它们可以降低凝固点和提高沸点。

they include freezing point depression and boiling point elevation.

因此，溶液相对于纯溶剂有着较低的凝固点以及较高的沸点，

So, solutions have both a lower freezing point and a higher boiling point than pure solvent,

and the more solute is present, the bigger the difference.

而且溶质越多，差别越大。

So, why do these properties change?

那么为什么这些特性会改变呢？

First of all, we need to understand that temperature is a measure

首先，我们需要理解，温度是测量粒子的平均动能。

of the particle's average kinetic energy.

The colder the liquid, the less of this energy there is,

液体温度越低，这种能量越少，

and the slower the molecules move.

而且水分子的运动速度也会变慢。

When a liquid freezes, the molecules slow down,

当液体结冰时，分子运动速度减慢，

enough for their attractive forces to act on each other,

速度慢到足以让它们的引力来进行相互作用

arranging themselves into a crystal structure.

自行地排成一个晶体结构

But the presence of solute particles gets in the way of these attractions,

但溶质粒子的存在阻碍了这些引力,

requiring a solution to be cooled down further before the arrangement can occur.

需要一种溶液在自行排列前冷却下来。

As for the boiling point, when a liquid boils,

至于沸点,在液体沸腾时,

it produces bubbles filled with its vapor,

它产生气泡并充满了蒸汽,

but for a bubble to form, the vapor pressure must become as strong

但要形成气泡 蒸汽压必须要变得很强

as the atmosphere constantly pushing down on the surface of the liquid.

因为大气不断压低表面的液体。

As the liquid is heated, the vapor pressure increases,

随着液体加热,蒸汽压会增强,

and when it becomes equal to the atmospheric pressure,

然后当蒸汽压和大气压是一样的时候

the bubbles form and boiling occurs.

就会产生气泡 发生沸腾的现象。

A solution's vapor pressure is lower than that of pure solvent,

溶液的蒸汽压比纯溶剂要低,

so it must be heated to an even higher temperature

所以溶液必须 加热到一个更高的温度

before it can match the strength of the atmosphere.

以此来和大气压强抗衡

As an added bonus, the pressure in the radiator

还有一个额外的好处,在散热器的压力

is kept above atmospheric pressure,

一直保持比大气压强的状态

raising the boiling point by another 25 degrees Celsius.

再高25摄氏度就可以达到沸点。

The solution commonly used for a car's cooling system

通常在冷却系统中采取的溶液是

is a 50/50 mixture of ethylene glycol and water,

用一种50/50的乙二醇和水的混合物,

which freezes at -37 degrees Celsius and boils at 106 degrees Celsius.

这种混合物的凝固点是零下37摄氏度，沸点是106摄氏度。

At the highest recommended proportion of 70 to 30,

在最受推崇的70/30的比例当中,

the freezing point is even lower at -55 degrees Celsius,

凝固点比零下55摄氏度还低，

and the boiling point rises to 113 degrees Celsius.

沸点将提高到113摄氏度。

As you can see, the more ethylene glycol you add,

由此看来，添加的乙二醇越多，

the more protection you get, so why not go even higher?

保护的能力越强， 所以为什么不用更多的乙二醇呢？

Well, it turns out you can have too much of a good thing

好吧，结果是你不可能有太多的好东西

because at higher proportions,

因为在很高的比例中，

the freezing point actually starts to go back up.

凝固点的温度实际上又会升高。

The properties of the solution head towards the properties of ethylene glycol,

溶液的属性和乙二醇的属性相对应,

which freezes at -12.9 degrees Celsius,

都在零下12.9摄氏度结冰，

a higher temperature than we attained with the solution.

比我们获得的溶液的温度高。

The solution flows through the engine, absorbing heat along the way.

该溶液流过发动机，在这过程中吸收热量。

When it reaches the radiator, it's cooled by a fan,

当溶液流到散热器的时候，溶液被风扇冷却下来，

as well as air rushing through the front of the car

还有从车头吹过来的风也使得溶液冷却下来

before returning to the hot engine compartment.

这些都是在溶液返回到热的引擎室之前发生的。

So, an effective and safe engine coolant

所以，一个高效安全的发动机冷却液

must have a high specific heat, a low freezing point, and a high boiling point.

必须要有一个高的比热容， 低的凝固点，还有一个高的沸点。

But instead of searching all over the world for the perfect liquid to solve our problem,

但是我们不需要满世界去找那种完美的液体来解决问题，

we can create our own solution.

我们自己可以创造溶液。