\Phase rule\ (Phys. Chem.)
A generalization with regard to systems of chemical
equilibrium, discovered by Prof. J. Willard Gibbs. It may be
stated thus: The degree of variableness (number of degrees of
freedom) of a system is equal to the number of components
minus the number of phases, plus two. Thus, if the components
be salt and water, and the phases salt, ice, saturated
solution, and vapor, the system is invariant, that is, there
is only one set of conditions under which these four phases
can exist in equilibrium. If only three phases be considered,
the system is univariant, that is, the fixing of one
condition, as temperature, determines the others.