Rock salt melts ice by lowering the freezing temperature of water. To understand how rock salt lowers the freezing temperature of water, you need to know a few things about the properties of water, ice and rock salt. Water is made of tiny molecules that are always in motion. The molecules move faster when the water is warm and slower when the water is cold. Ice forms when the temperature is cold enough for the molecules to slow down and form bonds with each other; normally, this happens at 32 degrees F (0 degrees C). The colder the water is, the faster the ice forms because the water molecules are moving so slowly they can't shake free of the bonds.
On the surface of ice, water is constantly freezing and melting as some water molecules break free of the bond, while others are captured. This happens at such a small level that you can't see it; you can just see the overall effect. If more molecules are melting than freezing, then you will see the ice begin to melt. If more molecules are freezing than melting, you will see the ice thicken and spread until there are no more water molecules left. At the freezing point of water, the number of molecules freezing and melting are the same.
When you dissolve rock salt in water, it separates into individual sodium and chloride ions. These ions are what lower the freezing point of water. When you pour rock salt on top of ice, the few water molecules that have broken free of the bond and are in liquid form are enough to dissolve a little bit of the rock salt. The rock salt inhibits the freezing process, so these molecules don't refreeze. As more water molecules break free, they dissolve more of the rock salt. This process continues until the rock salt is completely dissolved. Because the water molecules are melting, but not refreezing, the ice begins to melt. Rock salt inhibits freezing in two ways.
Water molecules move around in random directions, and whenever they come close enough to the ice, they freeze. When you dissolve rock salt into the water, the sodium and chloride ions dilute the water molecules. Instead of having just water moving around, you have sodium and chloride ions as well, so fewer water molecules bump into the ice and freeze. The ice melts because the number of water molecules breaking free of their bonds does not change, so more molecules are melting than freezing.
When you pour rock salt onto ice, the sodium and chloride ions interfere with the bonds between the water molecules that make up the ice. This makes it easier for the water molecules to break free of the bond and makes it harder for them to refreeze. This shifts the balance so more molecules are melting than freezing, and the ice begins to melt.
Rock salt lowers the freezing point of water to about 15 degrees F (-9 degrees C) in an average outdoor environment, instead of the normal 32 degrees F (0 degrees C). If the temperature falls below 15 degrees F, the water molecules move so slowly that they are able to form bonds and freeze, even with the interference of the salt. As the water freezes, the salt crystallizes and falls out of solution, until all of the water molecules are frozen. Other types of salt, such as calcium chloride are able to lower the freezing temperature of ice more than rock salt, and are used in areas where rock salt is not effective.
By Pamela Keene, eHow Contributor