Gibbs Phase Rule for One- and Two-Component Systems
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https://sites.google.com/a/learncheme.com/learncheme/simulations/cdf-files/GibbsPhaseRuleForOneAndTwoComponentSystems.cdf?attredirects=0&d=1
J. Willard Gibbs (ca. 1870) derived a simple rule that determines the number of degrees of freedom for a heterogeneous system. If a system in thermodynamic equilibrium contains  phases and  components, then the phase rule states that the number of degrees of freedom is given by . Degrees of freedom  represents the number of intensive variables (such as pressure, temperature, and composition) that can be varied arbitrarily over some finite range without changing the number of phases. The phase rule has been described as "pretty but powerful, qualitative yet exact".

Contributed by: S. M. Blinder (September 2010) 
Open content licensed under CC BY-NC-SA