CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 171
Chapter 7
Phase equilibria
7.1 Basic terms
7.1.1 Phase equilibrium.
When a system composed of two or more phases [see1.1.4] is in the state of thermodynamic
equilibrium [see1.4.1], we say that the system has attainedphase equilibrium.
Example
Liquid water and water vapour are in phase equilibrium at temperature 100◦C and pressure
101 325 Pa. Liquid water at 40◦C and ice at− 10 ◦C are not in phase equilibrium. An irreversible
process occurs during which ice heats and melts, and liquid water cools. In the state of thermo-
dynamic equilibrium, which the system attains after some time, there may be either a one-phase
system (all ice melts or all liquid water freezes), or a two-phase system (with phase equilibrium
existing between ice and liquid water at 0◦C).7.1.2 Coexisting phases.
When a system is in the state of phase equilibrium, we call its phasescoexisting phases. Co-
existing phases always have the same temperature and pressure. In the case of multicomponent
equilibria, the compositions of individual phases are identical only exceptionally [see7.6.6].