Preliminary discussion on reaction equilibria involving gases together with immiscible liquids and solids

Preliminary discussion on reaction equilibria involving gases together with immiscible liquids and solids 

We now consider chemical reactions in which one or more of the products or reactants is a solid or liquid. In the present chapter the discussion will be limited to cases where each of these solids or liquids is present in the system as a pure phase, i.e. when they do not take into solution appreciable amounts of the other components. Under these conditions the free energy of mixing, which has been shown to be an important part of the driving force of reaction, is limited to the gaseous phase. (The discussion of the case where there is an additional free energy of mixing in the condensed phases depends on a knowledge of the chemical potentials in solutions and will be deferred to Chapter 10.) 

Now every substance has a finite vapour pressure and therefore the reaction equilibrium may be thought of as being established in the gaseous phase, according to the principles already discussed. If the various solids or liquids which take part in the reaction are in phase equilibrium with their vapours, there will be reaction equilibrium throughout the entire system. 

For example, in the second of the above reactions, the condition of reaction equilibrium is 

The conditions of phase equilibrium for the oxide and carbonate between their solid phases and their vapours are 

---