noun Physical Chemistry.
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.Compare variance(def 4).
Origin of phase rule
First recorded in 1895–1900
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A rule used in thermodynamics stating that the number of degrees of freedom in a physical system at equilibrium is equal to the number of chemical components in the system minus the number of phases plus the constant 2. Also called Gibbs phase rule See also phase transition state of matter.
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