This entry contributed by David Rovnyak
In work on heterogeneous equilibria published in 1875-1876, J. Willard Gibbs Eric Weisstein's World of Biography derived a simple rule which determines the number of degrees of freedom for a heterogeneous system in equilibrium. The number of degrees of freedom for a system is the number of intensive variables (often taken as the pressure P, temperature T, and composition fraction) that may be arbitrarily specified without changing the number of phases.
If a system in equilibrium contains P phases and C components then the phase rule states that the number of degrees of freedom is given by
Goldschmidt's Mineralogical Phase Rule, Intensive Variable, Phase
References
Moore, W. J. Physical Chemistry, 5th ed. London: Longman, 1972.
