When roads freeze in the winter, the highway department is quick to spread salt on the roads to melt the ice. But the salt itself does not affect the rate of melting. No foreign substance will do that to ice. But salt can affect the freezing point. Adding salt water to ice brings the temperature at which water freezes down considerably. So as long as the external temperature is higher than the new freezing point, the ice will melt away. This is the same principle used when rock salt is used to cool an ice bath to less then freezing to create ice cream.
This works because the presence of the salt replaces some of the water molecules, meaning the ice and water can't exist in equilibrium at the freezing temperature. The ice does not come into contact with as many pure water molecules and therefore is unable to maintain the free exchange of molecules between the water and the ice. The result is melting ice, according to the Frostburg State University General Chemistry website.