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，那么你所在的城市應(yīng)該有一些除冰卡車，負(fù)責(zé)在公路、街道和人行道上撒鹽，以使路面的冰盡快融化。它的工作原理是什么？

首先，道路用鹽就是巖鹽，又名“石鹽”，即食鹽的天然形式。區(qū)別在于食鹽需要經(jīng)過漫長(zhǎng)的純化過程，而巖鹽則不需要。由于巖鹽中仍含有雜質(zhì)，因此它呈現(xiàn)棕色或灰色。

Why Salt？

為什么選擇鹽？

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，當(dāng)水溫達(dá)到32華氏度（0攝氏度）時(shí)，就會(huì)形成冰，這包括道路上的冰。道路用鹽通過一個(gè)稱為“冰點(diǎn)降低”的過程降低水的冰點(diǎn)。當(dāng)加入鹽后，水的冰點(diǎn)就會(huì)降低，因此鹽使水較難結(jié)冰。10%的鹽溶液在20華氏度（約零下6攝氏度）時(shí)結(jié)冰，20%的鹽溶液在2華氏度（約零下16攝氏度）時(shí)結(jié)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.

關(guān)鍵之處在于，要想使“冰點(diǎn)降低”發(fā)揮作用，道路上必須要有水，哪怕是極少的水。這就是為什么當(dāng)預(yù)報(bào)會(huì)出現(xiàn)冰雪天氣時(shí)，你經(jīng)常看到卡車提前在道路上撒鹽溶液（鹽和水的混合物）。如果在路面干燥的情況下把鹽撒到道路上去，那可能不會(huì)產(chǎn)生多大的作用。但是提前在道路上撒鹽溶液則有助于防止道路結(jié)冰，并有助于減少卡車隨后在道路上除冰所需的撒鹽量。

Pros and Cons

優(yōu)點(diǎn)和缺點(diǎn)

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.

巖鹽是使用最廣泛的道路除冰劑之一，但是對(duì)它也有一些負(fù)面的評(píng)價(jià)。巖鹽確實(shí)有它的局限性。如果路面的溫度低于15華氏度（約零下9攝氏度），鹽對(duì)冰面不會(huì)起任何作用。固態(tài)鹽根本無法進(jìn)入冰凍水的內(nèi)部結(jié)構(gòu)以發(fā)揮融化的作用。在這種情況下，通常會(huì)在冰面撒沙子以增加摩擦力。

巖鹽還會(huì)造成嚴(yán)重的環(huán)境問題，它含有會(huì)滲入地下和水中的鈉和氯。巖鹽因?yàn)闆]有經(jīng)過純化，所以含有鉛、鐵、鋁和磷等污染物。但是，巖鹽仍然是使用最廣泛、價(jià)格最便宜的除冰劑。盡管還有其他化學(xué)除冰劑，但是沒有哪一種是100%無風(fēng)險(xiǎn)的。

Word Bank

traction /'tr?k?n/ n. 附著摩擦力

leach /li?t?/ v. 過濾；濾

Nitrates leach from the soil into rivers.

contaminant /k?n't?m?n?nt/ n. 污染物