# phase rule

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Related to phase rule: Phase diagram

## 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, that is, both temperature and pressure are fixed.
Synonym(s): Gibbs phase rule
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References in periodicals archive ?
In general, correct solutions to the phase-mapping problem should follow the Gibbs phase rule, which specifies that the number of observed phases at a given chemical composition is no more than the number of chemical elements [N.sub.el]:
According to the phase rule, when a second element is dissolved in aluminum, we have an additional degree of freedom.
All TRC developments are built upon the algebraic constraints of the Gibbs phase rule, which specifies the number of free parameters necessary for a system to be thermodynamically well defined based on the number of chemical components and phases present.
From a thermodynamic perspective, consider Gibb's phase rule (10) which tells us, for equilibrium conditions, the number of degrees of freedom, f, available to a system of c components when there are p coexisting phases present.
The number of ways (degrees of freedom) in which temperature, pressure, or concentration can be varied in such cases can be expressed by a simple equation, which Gibbs called the phase rule. By the time he was through, Gibbs had left little more to do in what is now called chemical thermodynamics.
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