Two things happen when ice and water are placed in contact:

*Molecules on the surface of the ice escape into the water (melting), and

*Molecules of water are captured on the surface of the ice (freezing).

When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change on average (although there are short-term fluctuations at the surface of the ice).

The ice and water are said to be in dynamic equilibrium with each other. The balance between freezing and melting can be maintained at 0 degrees C, the melting point of water, unless conditions change in a way that favors one of the processes over the other.

The balance between freezing and melting processes can easily be upset. If the ice/water mixture is cooled, the molecules move slower. The slower-moving molecules are more easily captured by the ice, and freezing occurs at a greater rate than melting. Conversely, heating the mixture makes the molecules move faster on average, and melting is favored.

Adding salt to the system will also disrupt the equilibrium. Consider replacing some of the water molecules with molecules of some other substance. The foreign molecules dissolve in the water, but do not pack easily into the array of molecules in the solid. The total number of waters captured by the ice per second goes down, so the rate of freezing goes down. The rate of melting is unchanged by the presence of the foreign material, so melting occurs faster than freezing.

To re-establish equilibrium, you must cool the ice-saltwater mixture to below the usual melting point of water. For example, the freezing point of a 1 M NaCl solution is roughly -3.4 degrees C. Solutions will always have such a freezing point depression. The higher the concentration of salt, the greater the freezing point depression [1].

But won't any foreign substance cause a freezing point depression, according to this model? Yes! For every molecole of foreign particles dissolved in a kilogram of water, the freezing point goes down by roughly 1.7-1.9 degrees C. Sugar, alcohol, or other salts will also lower the freezing point and melt the ice. Salt is used on roads and walkways because it is inexpensive and readily available.

Note: Notice that when melting is complete, it can take a while for ice to begin to form again, even if the temperature is quite low. A "seed crystal" of ice must form by chance collisions before crystal growth really begins. Real liquids can exist for some time below their normal melting points.