When crushed ice combines with equal amounts and water, the water's temperature usually cools to about 32 degrees Fahrenheit, the melting point of ice, and the freezing point of water.
Adding salt to the icy water, however, actually cools the water below 32 degrees fahrenheit, and there are two reasons why this phenomenon occurs.
In this equilibrium state, it is just as likely for individual molecules from the liquid water to unite with the surface of the ice crystal as it is for molecules on the surface of the crystal to cross into the liquid water.
Dissolving salt into the ice water disrupts this equilibrium, because the separate sodium and chlorine ions, with their electrical charge, attracts molecules from the liquid water, so that the water molecules form a shell around the ions. As more the water moecules connect to the ions, there are fewer water molecules to combine with the ice.
Water molecules from the ice continue leaving the ice crystal as easily as before, however, to unite with the liquid water, thereby melting the ice. Since melting requires heat, and the heat comes from the water, the temperature of the water drops as it uses heat to melt the ice. As more salt is added, the water temperature will continue to drop as long as the solution still contains solid ice.
The same phenomenon occurs on snow or ice in the winter. Salt lowers the freezing point of ice, thereby converting the ice on roads and sidewalks into very cold water. Source: "We All Scream for Ice" The Washington Post, Column: PHENOMENA Wednesday, July 9, 1997 ; Page H08