Salt is put on icy sidewalks to lower the temperature required for the ice to start to melt. Salt can lower both the melting and freezing points of water. Adding salt to water can lower the water's freezing temperature from 32 degrees to 20 or even 2 degrees Fahrenheit.
When salt is placed on an icy sidewalk, the ice crystals around the grain of salt melt first. The melting process then spreads outwards, causing the ice on the sidewalk to melt into water. The effectiveness of salt on icy sidewalks diminishes when the temperature of surface is lower than 15 degrees Fahrenheit.