**Gibbs Phase Rule for One- and Two-Component Systems**

J. Willard Gibbs (ca. 1870) derived a simple rule that determines the number of degrees of freedom for a heterogeneous system. If a system in thermodynamic equilibrium contains phases and components, then the phase rule states that the number of degrees of freedom is given by . Degrees of freedom represents the number of intensive variables (such as pressure, temperature, and composition) that can be varied arbitrarily over some finite range without changing the number of phases. The phase rule has been described as "pretty but powerful, qualitative yet exact". |