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Meaning of phase rule

phase' rule"

Pronunciation: [key]
— Physical Chem. Physical Chem.
  1. a law that the number of degrees of freedom in a system in equilibrium is equal to two plus the number of components less the number of phases. Thus, a system of ice, melted ice, and water vapor, being one component and three phases, has no degrees of freedom. Cf.(def. 4).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.