ABSTRACT
This chapter considers condensed phases that are of pure species—that is, are of fixed composition. Many practical systems occur in this category: for example, the reaction of pure metals with gaseous elements to form pure metal oxides, sulfides, halides, and so on. The following two facts facilitate a convenient definition of the standard state of species occurring as condensed phases: A pure species occurring as a condensed phase exerts a unique saturated vapor pressure at the temperature T; and the dependence of the Gibbs free energy of a condensed phase on pressure is negligibly small. This standard state is simply the pure species in its stable condensed state at the temperature T. Using this standard state, the equilibrium constant for a reaction involving pure condensed phases and a gas phase can be written in terms of the partial pressures of those species which occur only in the gas phase.
