(Note: If you're already familiar with chemical potentials, you may be interested in this alternative thermodynamic explanation.)
Two things happen when ice and water are placed in contact:
Molecules on the surface of the ice escape into the water
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°C, the melting point of water, unless conditions change in a way that favors one of the processes over the other.
If you don't see the animation above, a nonanimated version is available; or you can download the free Flash plugin from Macromedia.
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. You can see a demonstration of this by clicking on the temperature in the animation and setting it to a lower value (say, -10).
Conversely, heating the mixture
makes the molecules move faster on average, and melting is favored. Reset the animation and then enter a higher value for the temperature (say 10) and watch what happens.
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.
Try hitting the "Add Solute" button in the animation above.
Notice that there are fewer water molecules on the liquid side because the some of the water has been replaced by salt. 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
That's why salt melts ice.
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°C. Solutions will always have such a freezing point depression.
The higher the concentration of salt, the greater the freezing point depression .
But won't any foreign substance cause a freezing point depression, according to this model? Yes! For every mole of foreign particles dissolved in a kilogram of water, the freezing point goes down by roughly 1.7-1.9°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.
It is important to realize that freezing point depression occurs because the concentration of water molecules in a solution is less than the concentration in pure water. The nature of the solute doesn't matter. One might expect from the diagram above that solutes with large molecules are better at blocking water molecules travelling towards the surface of the ice. The hypothesis that solutes with large molecules cause a larger freezing point depression than those with smaller molecules is not in accord with experimental data! The misconception arises because the diagram can't be drawn to scale; the size of the molecules is very small compared to the distance between them.
Phase map for salt water. Drawn from a diagram by R. E. Dickerson (Note 3)
As ice begins to freeze out of the salt water, the fraction of water in the solution becomes lower and the freezing point drops further. This does not continue indefinitely, because eventually the solution will become saturated with salt.
The lowest temperature possible for liquid salt solution is -21.1°C.
At that temperature, the salt begins to crystallize out of solution (as NaCl·2 H2O), along with the ice, until the solution completely freezes.
The frozen solution is a mixture of separate NaCl·2H2O crystals and ice crystals, not a homogeneous mixture of salt and water.
This heterogeneous mixture is called a eutectic mixture.
References and Notes
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 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.