If you live in a city that gets lots of snow and ice, then you're familiar with road salt. Your city and local government likely has several de-icing trucks that spread road salt on the highways, streets and sidewalks to melt the ice. But how exactly does it work?

如果你生活在一座冰雪之城，那么你肯定对道路防结冰用盐不陌生。你所在的城市和政府应该会有几辆除冰车负责在高速公路、街道和人行道上撒盐，以使路面的冰尽快融化。那么这背后的工作原理到底是什么？

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

首先，道路用盐就是石盐，也被称为岩盐，即食盐的天然形式。区别在于食盐经过了漫长的纯化工艺，而岩盐则没有。另外，由于岩盐中还含有杂质，因此其呈现棕色或灰色。

Table salt：食盐

Why Salt?

为什么选择盐？

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 it the salt makes it more difficult for water to freeze. A 10-percent salt solution freezes at 20 degrees Fahrenheit (-6 Celsius), and a 20-percent solution freezes at 2 degrees Fahrenheit (-16 Celsius).

当水温达到32华氏度（或0摄氏度）时，就会形成冰，这也包括马路上的冰。道路用盐通过一种被称为冰点降低的工艺来使水的冰点降低并从而发挥作用。当加入盐后，水的冰点就会降低，因此盐使水变得较难结冰。浓度为百分之十的盐溶液会在20华氏度（零下6摄氏度）时结冰，而浓度为百分之二十的盐溶液则会在2华氏度（零下16摄氏度）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. That's why you often see trucks pre-treat roads with a brine solution (a mixture of salt and water) when ice and snow is forecast. If the roads are dry and the DOT simply puts down road salt, it likely won't make much of a difference. But pre-treating 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，马路上必须要有水，哪怕是极少的水。这也是为什么当预报会出现冰雪天气时，你会经常看到卡车会提前在路面上撒盐溶液（盐和水的混合物）。如果在路面干燥的情况下把道路用盐撒上去，那很可能不会产生任何作用。但是提前在马路上撒盐溶液则有助于防止结冰，并从而减少随后的撒盐量。

Pros and Cons

优点和缺点

Rock salt is one of the most widely used road de-icers, but it's not without critics. For one, rock salt does have its limits. If the temperature of the roadway is lower than about 15 degrees F (- 9 C), the salt won't have any effect on the ice. The solid salt simply can't get into the structure of the frozen water to start the dissolving process. In these cases, the DOT typically spreads sand on top of the ice to provide traction.

道路用盐是适用范围最广的道路去冰材料之一，但是它也有一些负面的评价。首先，道路用盐有局限性。如果路面的温度低于15华氏度（零下9摄氏度），盐对冰就不会有任何作用。固态盐根本无法进入冰的内部以发挥融化的作用。在这种情况下，运输部往往会把沙子洒在冰面上以增加摩擦力。

Rock salt also has major environmental issues, including the sodium and chlorine that leaches into the ground and water. And as we mentioned earlier, because rock salt isn't purified and contains contaminants — including lead, iron, aluminum and phosphorus — when it's spread, these are spread as well. However, rock salt still remains the most widely used and affordable de-icers available. And while there are other chemical de-icers, too, none are 100 percent risk free.

道路用盐还有非常严重的环境问题，包括钠和氯会渗入到地面和水中。正如我们早些时候提到的，因为岩盐没有经过纯化，因此还会含有污染物，比如铅、铁、铝和磷-当撒下道路用盐时，这些污染物也会被一同撒下。但是，道路用盐仍然是应用最广以及最经济的去冰材料。尽管还有其他化学类的去冰材料，但是没有哪一种是百分百安全的。

原文来源：HowStuffWorks

编辑：上海译锐翻译