The reason salt causes ice to melt is that a water solution, and the dissolved salt has a lower freezing point than a pure substance like plain water.
The addition of salt to ice dissolves the film of liquid water on the surface of the ice. This lowers the film’s freezing point below the ice’s temperature. The salty water solution begins to melt the ice and then creates more liquid water.
However, using salt to de-ice a sidewalk when the temperature is below 0 degrees Fahrenheit or -17.77 degrees Celsius does not melt the ice.Learn more about States of Matter