Why does salt melt ice?
Salt melts ice by lowering the freezing point of water. When salt dissolves in the thin layer of water on ice, the salty water needs a lower temperature to freeze, so the ice melts.
Even very cold ice usually has a tiny film of liquid water on its surface. Salt dissolves into this film, creating brine. Pure water freezes at 0 degrees Celsius, but salty water can stay liquid at much lower temperatures. As more ice melts, more salt dissolves, and the process continues.
This effect is called freezing point depression. Road salt, usually sodium chloride, works well down to about minus 9 degrees Celsius. In colder conditions, it becomes much less effective, so cities use other chemicals, such as calcium chloride or magnesium chloride, which work at lower temperatures.
Downsides of road salt
Salt helps keep roads and sidewalks safer in winter, but it can damage cars, bridges and concrete by speeding up rust. It also washes into rivers, lakes and soil, harming plants and wildlife. Some places use sand for grip or mix salt with beet juice to reduce how much is needed.
Did You Know?
The same principle is used to make homemade ice cream. Salt added to ice around the ice cream container makes it colder than 0 degrees, freezing the cream faster.