Why Is Salt Used to Melt Ice on the Roads in Winter?为什么冬天要用盐来融化路上的冰？

If you live in a city that gets lots of snow and ice， then your city likely has several de-icing trucks that spread road salt on the highways， streets and sidewalks to melt the ice. How exactly does it work？

First， road salt is simply halite—rock salt—which is table salt in its natural form. The difference is that table salt goes through a lengthy purification process， while rock salt does not. And because rock salt still has impurities， its brown or gray in color.

如果你生活在一座冰雪之城，那么你所在的城市应该有一些除冰卡车，负责在公路、街道和人行道上撒盐，以使路面的冰尽快融化。它的工作原理是什么？

首先，道路用盐就是岩盐，又名“石盐”，即食盐的天然形式。区别在于食盐需要经过漫长的纯化过程，而岩盐则不需要。由于岩盐中仍含有杂质，因此它呈现棕色或灰色。

Why Salt？

为什么选择盐？

Usually， ice forms when the temperature of water reaches 32 degrees Fahrenheit （0 degrees Celsius）， and that includes ice on roadways. Road salt works by lowering the freezing point of water via a process called freezing point depression. The freezing point of the water is lowered once the salt is added， so the salt makes it more difficult for water to freeze. A 10-percent salt solution freezes at 20 degrees Fahrenheit （about-6 degrees Celsius）， and a 20-percent solution freezes at 2 degrees Fahrenheit （about-16 degrees Celsius）.

通常情況下，当水温达到32华氏度（0摄氏度）时，就会形成冰，这包括道路上的冰。道路用盐通过一个称为“冰点降低”的过程降低水的冰点。当加入盐后，水的冰点就会降低，因此盐使水较难结冰。10%的盐溶液在20华氏度（约零下6摄氏度）时结冰，20%的盐溶液在2华氏度（约零下16摄氏度）时结冰。

The key is， there has to be at least a tiny bit of water on the road for freezing point depression to work. Thats why you often see trucks pretreat roads with a brine solution （a mixture of salt and water） when ice and snow is forecast. If the roads are dry and road salt is put down， it likely wont make much of a difference. But pretreating with a brine solution can help ice from ever forming， and will help reduce the amount of road salt trucks will need to spread to de-ice later.

关键之处在于，要想使“冰点降低”发挥作用，道路上必须要有水，哪怕是极少的水。这就是为什么当预报会出现冰雪天气时，你经常看到卡车提前在道路上撒盐溶液（盐和水的混合物）。如果在路面干燥的情况下把盐撒到道路上去，那可能不会产生多大的作用。但是提前在道路上撒盐溶液则有助于防止道路结冰，并有助于减少卡车随后在道路上除冰所需的撒盐量。

Pros and Cons

优点和缺点

Rock salt is one of the most widely used road de-icers， but its not without critics. Rock salt does have its limits. If the temperature of the roadway is lower than about 15 degrees Fahrenheit （about -9 degrees Celsius）， the salt wont have any effect on the ice. The solid salt simply cant get into the structure of the frozen water to start the dissolving process. In these cases， sand is typically spread on top of the ice to provide traction.

Rock salt also has major environmental issues， including the sodium and chlorine that leaches into the ground and water. Rock salt isnt purified and contains contaminants including lead， iron， aluminum and phosphorus. However， rock salt still remains the most widely used and affordable de-icer available. And while there are other chemical de-icers， too， none are 100 percent risk free.

岩盐是使用最广泛的道路除冰剂之一，但是对它也有一些负面的评价。岩盐确实有它的局限性。如果路面的温度低于15华氏度（约零下9摄氏度），盐对冰面不会起任何作用。固态盐根本无法进入冰冻水的内部结构以发挥融化的作用。在这种情况下，通常会在冰面撒沙子以增加摩擦力。

岩盐还会造成严重的环境问题，它含有会渗入地下和水中的钠和氯。岩盐因为没有经过纯化，所以含有铅、铁、铝和磷等污染物。但是，岩盐仍然是使用最广泛、价格最便宜的除冰剂。尽管还有其他化学除冰剂，但是没有哪一种是100%无风险的。

Word Bank

traction /'tr?k?n/ n. 附着摩擦力

leach /li?t?/ v. 过滤；滤

Nitrates leach from the soil into rivers.

contaminant /k?n't?m?n?nt/ n. 污染物