We know that most substances can exist in more than one phase. For example: solid, liquid and gas. Some substances even have more than one solid phase. Carbon exists as graphite, diamond and a whole encyclopaedia of forms related to C60; tin exists as white tin and grey tin; and so on. A great amount of information on the phases of a substance can be summarized on one diagram called a "phase diagram."

Applications:

• It applies to physical as well as chemical phase reaction.
• It provides a convenient basis for classification of equilibrium of states with the help of phases, component and degree of freedom.
• It applies to microscopic systems.
• It indicates that different systems having the same degree of freedom behave in a similar fashion.
• The phase rule is used to describe systems of one component as well as multi-component systems.
• For one component system of water, the phase rule is applied and the phase diagram gives a lot of information like the triple point, sublimation temperature, boiling point and freezing point.

The phase diagram of solvents can be used to determine the parameters useful distillation procedures.

For a single-component system, such as a sample of pure water, the composition is, by default, fixed and C = 1.

If the substance is present as only one phase, e.g. a solid, then P = 1. The phase rule therefore shows that

F=C+2-P=1+2-1=2

and the system is therefore di-variant with two degrees of freedom. We could, for example, change both the temperature and the pressure independently and the substance would remain as a solid.