Conventional thermodynamic expression predicts that the isobaric heat capacity decreases with increasing pressure. In model
calculations, heat capacity increases with pressure, decreases, or remains insensitive to pressure, depending on the model
applied. The expression cannot be applied to the gases, but experimental data on gases show evidently that heat capacity increases
Considering the change in enthalpy along two different paths with identical starting and ending points, we derive new expression
dCP/dP=αV, where α is the volume coefficient of thermal expansion and V is the molar volume. The expression predicts the increase in CP with pressure and can be applied to gases. The test of the new expression against accurate literature data on the heat capacity
of air, gaseous and liquid, demonstrates its validity.