Phase rule

General principle in thermodynamics regarding pVT systems in equilibrium

In thermodynamics, the phase rule is a general principle governing multi-component, multi-phase systems in thermodynamic equilibrium. For a system without chemical reactions, it relates the number of freely varying intensive properties (F) to the number of components (C), the number of phases (P), and number of ways of performing work on the system (N): F = N + C − P + 1 {\displaystyle F=N+C-P+1} Examples of intensive properties that count toward F are the temperature and pressure. For simple liquids and gases, pressure-volume work is the only type of work, in which case N = 1.

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