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phase rule

An expression of the relationships existing between systems in equilibrium: P _ V = C _ 2, where P is the number of phases, V the variance or degrees of freedom, and C the number of components; it also follows that the variance is, V = C _ 2 -P. For H2O at its triple point, V = 1 _ 2 -3 = 0, i.e., both temperature and pressure are fixed.

Synonym: gibb's phase rule.