Why do we put salt on icy roads?

In places with cold winters, it is a familiar scene: trucks spreading salt across roads, or people scattering it on icy sidewalks. Salt is one of the oldest and most widely used tools for fighting ice. But salt is not warm, and it does not have any heat of its own to give. So how can sprinkling ordinary salt actually make solid ice turn to liquid? The answer comes from a neat principle of chemistry.

Pure water freezes at zero degrees Celsius. That is the temperature at which liquid water turns into solid ice, and also the temperature at which ice melts back into water. This freezing point is not fixed forever, though. It can be changed. If you mix another substance into water, you can actually lower the temperature at which that water will freeze. This effect is known as freezing point depression, and it is the key to how road salt works.

When salt is added to water, it dissolves and spreads out among the water molecules. This makes it harder for the water molecules to lock together into the orderly structure of ice. As a result, the salty water has to get much colder than zero degrees before it will freeze. On an icy road, there is always a thin film of liquid water on the surface of the ice. When salt mixes into this film, it lowers the freezing point there, so the ice in contact with it melts. This is also why spreading salt before a storm helps prevent ice from forming in the first place.

Road salt is effective, but it is not magic and has limits. If the temperature drops too low, ordinary salt stops working well, because even very salty water will eventually freeze when it gets cold enough. In those cases, other types of salt that work at lower temperatures may be used instead. Salt also has downsides: it can be harsh on metal, causing vehicles to rust, and large amounts can harm soil, plants, and water supplies. Because of this, cities try to use salt carefully, balancing safe roads against its costs.

This article was written using information from Wikipedia.