Water freezes and melts at zero degree. Unless the conditions are changed water does not freeze or melt, and a balance is maintained. Now if the temperature is lowered ice/water mixture will start forming ice and if temperature is increased ice/water mixture starts melting.
If we add salt to the mixture the conditions are changed. Now some of the water molecules has been replaced by salt molecules so rate of capturing water molecules decrease and less water molecules reach to the ice surface where it can be easily captured by the ice causing the ice/water mixture to lower its freezing point
Molecules in ice moves slowly and in water moves comparatively fast. When heat is removed the temperature decreases causing water to freeze and vice versa. Salt and water molecules stick with each other when we add salt to the mixture. Now additional heat is to be removed for the freezing of this combination of salt and water. Hence the freezing point is lowered and ice starts melting. Similarly boiling point also increases with the addition of salt in water. More heat is required for the combination of salt-water molecules to move fast.
To prevent freezing in the cars in winter same principle is applied by antifreeze. Similarly highway department spreads salt over the snow scattered on the roads. The snow which is in direct contact with salt grains start melting, hence enhancing the process.
Not only salt, anything that dissolves in water has the same affect.
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