Why should salt be sprinkled on the road in rainy and snowy days?

Before answering this question, let's first understand some physical and chemical characteristics of salt (main component Nacl) used for snow melting:

Solubility. Salt is soluble in water, which is a very common physical phenomenon.

(2) Moisture absorption. The surface of salt is usually surrounded by a film of saturated salt solution. When the vapor pressure of this membrane solution is lower than the partial pressure of water vapor in the air, the salt absorbs the moisture in the air and deliquesces; On the contrary, when it is higher than the partial pressure of water vapor in the air, it will dry. Of course, this phenomenon only happens when the salt is exposed to the air, and the salt sealed in the packaging bag and isolated from the surrounding air will not deliquesce;

③ Low freezing point. When I say freezing point here, I don't actually mean the melting point of pure salt (Nacl) crystal, because it can only be melted if it is heated to thousands of degrees Celsius without water. The freezing point mentioned here refers to the freezing point of salt solution. Experiments show that if the salt solution contains 20% salt, its freezing point will be reduced to-65438 06℃.

There are many other characteristics of salt, but to explain this problem, all of them are similar:) In addition, by the way, the composition of snow is mainly solid water-small ice crystals. No matter how low the temperature is, ice will sublimate, so water vapor will be scattered around the snow.

When we sprinkle salt on the snow, salt absorbs the water vapor on the snow surface by its moisture absorption characteristics, thus making itself more and more "wet" and the film of salt solution becomes more and more obvious. I think at this time, the solution layer kept "invading" the surrounding snow layer like sparks, and finally the snow was destroyed!