Phase rule

[Sarah]

A generalization with regard to systems of chemicalequilibrium, discovered by Prof. J. Willard Gibbs. It may be statedthus: The degree of variableness (number of degrees of freedom) of asystem is equal to the number of components minus the number ofphases, plus two. Thus, if the components be salt and water, and thephases salt, ice, saturated solution, and vapor, the system isinvariant, that is, there is only one set of conditions under whichthese four phases can exist in equilibrium. If only three phases beconsidered, the system is univariant, that is, the fixing of onecondition, as temperature, determines the others.