4 - Phase equilibria

Abstract

A phase is a homogeneous part of a system. Any heterogeneous system comprises two or more phases. Any mixture of gases or vapors is a one-phase system. Mixtures of two or more completely miscible liquids or solids are also one-phase systems, but mixtures of two partially miscible liquids or a heterogeneous mixture of two solids are two-phase systems, and so on. The three variables that can be considered in a system are temperature, pressure, and concentration. The number of these variables that may be changed in magnitude without changing the number of phases present is called the number of degrees of freedom. It may be said that the physical nature of a system can be expressed in terms of phases and that the number of phases can be changed by altering one or more of three variables temperature, pressure, or concentration. The chemical nature of a system can be expressed in terms of components, and the number of components is fixed for any given system.