Ex: Why a gas has two principal specific heat capacities? Or Gases have two specific heats, but the solids and liquids possess only one specific heat. Why?
Solution: When the gases are heated, there occurs an appreciable change in their volume. So energy is required for expansion and we have two specific heats one at constant pressure and the other at constant volume. In case of solids and liquids, expansion is negligible, so they have only one specific heat i.e., at constant volume.