Equilibrium changes at constant mass

If we have a two-phase one-component system and we carry out an equilibrium change during the course of which the masses of the two phases remain unchanged, then the change is called a constant-mass equilibrium change. 

On dividing this expression by n, the number of moles in the first phase, we have  

For all temperatures sufficiently far from the critical temperature, we have in general 

Hence the molar volume of the vapour phase decreases with increase of temperature that is to say that the density of the saturated vapour increases with temperature. On the other hand, the molar volume of the liquid increases with temperature, or its density diminishes. 

Finally we consider the heat capacity in the course of an equilibrium transformation at constant mass. By virtue of (2.13), (19.78) and (4.42) we have for such a transformation 

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