If there is snow on the road, you can sprinkle salt to solve it. What is the principle?

In winter, northerners must have seen sanitation workers and highway departments sprinkle salt on the snow after heavy snow to prevent the snow from freezing at low temperature after melting and ensure traffic safety. The first purpose of sprinkling salt on snow is to make it melt quickly. This is because salt releases a lot of heat when it is dissolved in water, which accelerates the melting of snow. Why can salt accelerate the melting of snow, but the melted snow is not easy to freeze? Why putting salt on snow can prevent it from freezing. From the molecular point of view, salt is composed of potassium chloride. When dissolved in water, it will become free chloride ions and potassium ions, both of which will destroy the network formed when water freezes and prevent snow from freezing.

When salt is dissolved in water, the freezing point of brine will also decrease, about-10℃, while the freezing point of ordinary water is about 0℃. It can be seen that salt water does reduce the freezing point temperature, making it difficult for snow to freeze at low temperature after melting. Although sprinkling salt on the snow in winter has such advantages, at present, we are reducing the practice of sprinkling salt on the snow in winter, because the snow dissolved in salt does great harm to the green vegetation on both sides of the road and pollutes the groundwater, which is still very harmful. At present, snow removal has tended to be done by construction machinery. When snow is sprinkled with salt, sodium chloride quickly dissolves in the water molecules of snow, changing the freezing point of the snow surface.

Because the freezing point of water is zero, and the freezing point of salt water is ten or twenty degrees, the ground sprinkled with salt will not freeze because of a few degrees below zero. Its chemical principle is that sodium chloride adheres to the surface of snow and dissolves quickly in water, which ionizes sodium chloride into sodium ions and chloride ions, destroying the crystal structure of water and preventing it from freezing. Sprinkling salt on the road to melt the ice on the ground takes advantage of this characteristic of salt water. It is difficult to drive when the road is icy in winter. Traffic police and traffic card departments can effectively prevent the road surface and airport runway from freezing by sprinkling salt. In addition, salt is also a cheap and pollution-free substance, so salt is generally used as a snow melting agent for pavement.

However, the salt used for pavement also has the limitation of lowering the freezing point, and the effect of supercooling weather is not good. Long-term use of salt has the defect of damaging the pavement. At the same time, after adding salt to road traffic, sewage will splash snowmelt salt water. Although the car body has been treated with anti-corrosion to isolate the contact between metal materials and the outside world, the paint on the car body surface, such as the cracks in the car chassis, will inevitably be splashed by salt water, and the parts of the car body surface such as the palms and chassis will be corroded by the direct contact between metal and the outside world, which is one of the reasons why most people use special snow removal vehicles now.