A gas has two specific heats , whereas a liquid and a solid have only one . Why ?

Explanation

Coefficient of expansion of a gas is large .Therefore , heat supplied to a gas is spent not only in raising its temperature but also in its expansion . Thus ,specific heat of gas is defined either at constant volume or at constant pressure .
However in case of solids and liquids , the coefficient of expansion is very small .Thus the amount of heat spent in their expansion is negligible .