# phase equilibria

In thermodynamics, phase equilibria are equilibria between
substances in different phases. Phase
diagram, basic to engineering, metallurgy, and mineralogy, are empirical
graphs showing what phases exist at different pressures and temperatures.
The phase rule, deduced by Josiah Gibbs, states
that, for a closed system, *F* = *C* - *P* + 2, where *P* is the number of phases, *C* the number of independent
chemical components, and *F* the number of degrees
of freedom, i.e., the number of variables (pressure, temperature, composition)
whose values must be specified to define the system.