What is "phase rule"

Phase rule \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.

n. (context physics English) the rule which states that the number of degrees of freedom in a system at equilibrium equals the number of components minus the number of phases plus 2

Gibbs' phase rule was proposed by Josiah Willard Gibbs in his landmark paper titled On the Equilibrium of Heterogeneous Substances, published from 1875 to 1878. The rule applies to non-reactive multi-component heterogeneous systems in thermodynamic equilibrium and is given by the equality

F = C − P + 2
where F is the number of degrees of freedom, C is the number of components and P is the number of phases in thermodynamic equilibrium with each other.

The number of degrees of freedom is the number of independent intensive variables, i.e. the largest number of thermodynamic parameters such as temperature or pressure that can be varied simultaneously and arbitrarily without affecting one another. An example of one-component system is a system involving one pure chemical, while two-component systems, such as mixtures of water and ethanol, have two chemically independent components, and so on. Typical phases are solids, liquids and gases.