Systems of more than one phase

When a system consists of more than one phase, each phase may be considered as a separate system within the whole. The thermodynamic parameters of the whole system may then be constructed out of those of the component phases. If the interaction between the phases were restricted to energy exchange (flow of heat and performance of work), then application of thermodynamics to the whole system would not lead to any essentially new results. However, if we allow new degrees of freedom within the system, such as mass transport between phases or chemical reaction between constituents, the conditions for thermodynamic equilibrium (derived in section 7.4) do lead to new results which are related to the restrictions which equilibrium places on the new degrees of freedom. In this chapter, we shall restrict ourselves to considering systems whose chemical composition is uniform (for example, systems of one component) but in which more than one phase is present. For simplicity, we shall again develop the general results for a system subjected to work by hydrostatic pressure only.

The condition for equilibrium between phases

Let us first consider a one-component system of two phases maintained at constant pressure and temperature (Fig. 10.1). This might be a liquid in contact with its vapour. If we ignore any possible surface effects at the interface, both temperature and pressure will be uniform throughout.