Why does salt melt ice?
(Lansing State Journal, March 4, 1992)
Water becomes ice at a temperature of 32 degrees Fahrenheit (0 degrees Celsius), which we call the freezing point of water. Adding salt to water lowers the freezing point, thus preventing the formation of ice.
The primary difference between water and ice is the speed at which the molecules move. In water, the molecules move more rapidly than in ice. The temperature of the water is changed by adding or removing heat. As heat is removed, the water molecules slow down. At the freezing point, the motion of the molecules is slow enough that the water becomes ice.
When salt is added to water, the salt and water molecules stick together, <ore heat then must be removed to slow down and separate the combined salt and water molecules. The removal of the additional heat is required to freeze the salt-water combination. This means that the temperature at which it freezes is below that of pure water.
Antifreeze uses the same principle to keep the water in your car from freezing in the winter. Spreading salt over roads prevents melting snow from freezing into ice and causing hazardous road conditions.
For similar reasons, the addition of salt to water increases the boiling point of the water: more heat must be added to make the larger combined salt plus water molecule move fast enough to boil than must be added to make plain water boil. The result is that the temperature of the boiling salted water is higher than that of plain water.
Salt is not the only substance which lowers the freezing point of water. In fact, anything that can be dissolved in water will have the same effect.