Gibbs’ phase rule applied to phase diagrams and transformations

Abstract

The phase rule is derived using only potentials as variables. It is then modified in order to apply to phase diagrams obtained by sectioning and by introducing molar quantities as variables. When modified for constant composition, the phase rule takes its original form but now applies only when the predicted variance is 2 or less. When the composition is kept constant, one must consider compositional constraints, but this is not necessary when potential variables are considered. The character of a phase transformation can be studied by considering the character of the corresponding one-di-mensional phase diagram.