Gibbs Phase Rule

A thermodynamic counting rule that gives the number F of independent intensive degrees of freedom (variables such as temperature, pressure, composition) for a system at equilibrium as F = C − P + 2, where C is the number of chemically independent components and P the number of phases present; the +2 corresponds to temperature and pressure and is reduced by one for condensed systems where pressure

A thermodynamic counting relation that gives the number of independent intensive degrees of freedom F for a macroscopically homogeneous, multicomponent, multiphase system at equilibrium: F = C − P + 2, where C is the number of chemically independent components and P the number of coexisting phases (for non‑reactive systems in absence of external fields).

A counting relation that gives the number of independent intensive variables (degrees of freedom F) that can be varied in a heterogeneous system at thermodynamic equilibrium: F = C − P + 2 for a non-reacting system, where C is the number of independent components and P the number of coexisting phases; the formula is modified when chemical reactions or external constraints are present.