The Phase Rule as a Tool

Phase diagrams are both constructed and interpreted in accordance with certain principles of physical chemistry embodied in what is known as the Gibbs phase rule. A working knowledge of the phase rule is essential for the effective use of phase diagrams. Such a working knowledge is very simple to acquire, since the phase rule is a very simple statement about the relationships of three variables. Like many rules, the phase rule conceals in its simplicity a very subtle, brilliant, and powerful chain of logical reasoning, and it is possible, perhaps even desirable for advanced students, to analyse phase diagrams from first principles without using the rule as a crutch. We shall use the phase rule as a tool, however, and the purpose of this chapter is to provide some fundamental definitions and a very rudimentary sketch of the principles upon which the phase rule rests. As we shall develop and use it, the phase rule is a part of what is called classical thermodynamic5, the counterpart of which is statistical mechanics.