The Phase Rule, given by Willard Gibbs, is defined as, in heterogeneous systems, if equilibrium between phases are not influenced by gravity, magnetic & electrical forces, but are influenced only by pressure, temperature and concentration, then the number of degree of freedom (F) of the system is related to number of components (C) and number of phases (P) by the following phase rule equation

F=C-P+2

1. Phase

It is defined as “any homogenous, physically distinct and mechanically separable portion of the system, which is separated from other such parts of the system by definite boundary surfaces".

Examples:

• Water consists of three phases: Ice (S) | Water (L) | Water Vapour (g)
• A gaseous mixture, miscible in all proportions, will constitute only one phase.
• The two immiscible liquids (benzene and water) will form two liquid phases.
• The two miscible liquids (i.e. alcohol and water) will form one separate phase.
• A solution of a substance in a solvent consists of one phase only e.g.; Sugar solution in water.

• A heterogeneous mixture like: CaCO3 (s)⟶CaO(s)+CO_2(g), consists of three phases (i.e. two solids and one gaseous)

   

2. Component

It is defined as, "the smallest number of independent variable constituents, by the means of which the composition of each phase can be expressed in the form of chemical equation".

Example:

In the water system, the chemical composition of all the three phases is H20. Hence, it is one component system.

Ice(s)⇌Water(l)⇌Vapour(g)

• The sulphur system consists of four phases, rhombic, monoclinic, liquid and vapour, the chemical composition of all the phases is ‘S’. Hence, it is also one component system.

• A saturated solution of NaCl consists of solid NaCl, NaCl solution and water vapour. The chemical composition of all the three phases is NaCl & H20. Hence it is a two component system.

   

Examples:

1. In the water system

Ice(s)⇌Water(l)⇌Vapour(g)

Three phase will be in equilibrium only at particular temperature and pressure. The system is, zero variant, or non-variant or invariant and has no degree of freedom.

1. For a system consisting of water in contact with its vapour.

Water(l)⇌Vapour(g)

We must state either temperature or pressure to define the system completely. Hence, the degree of freedom is one (or) the system is invariant.

1. For a system consisting of

NaCl(s)⇌NaCI⋅Water(aq)⇌Water vapour(g)

We must state either temperature or pressure. Hence the system is univariant (degree of freedom is one).

For a gaseous mixture of N2 and H2, we must state both the temperature and pressure. Hence, the system is bi-variant (degree of freedom is two).

3.Degree of Freedom:

The degrees of freedom in a particular situation is the number of independent co-ordinates required to completely specify the state of a system. The number of degrees of freedom is the number of independent intensive variables, i.e. the largest number of properties 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.

 For example a single particle can absorb energy to increase its kinetic energy along three coordinate axes. Therefore, the degrees of freedom of a monatomic ideal gas atom is three.