When salt is placed on ice , the ice melts. Why?

A block of ice left undisturbed is in a state of thermal
equilibrium. There is a portion of the ice that melts to form water and at the same time an equal
amount of water solidifies to form ice. This establishes a state of equilibrium with the net
amount of ice remaining constant unless there is a change in the thermodynamic
conditions.

The addition of salt disturbs the equilibrium. Salt
dissolves in the ice that melts to water. Water, with ice dissolved in it, has a lower point of
freezing than pure water. Generally this applies to any solute that is added to water. All forms
of dissolved impurities in pure water decrease its point of
freezing.

This lowering of the freezing point does not allow an
equal amount of water to freeze again as the amount of ice that melts to form water. Hence salt
is able to melt ice.