Method of construction

The predominance area diagram depends on the system and temperature. In a two dimensional diagram, temperature is fixed. These are the equilibrium diagrams and hence we have to consider all the phases which can form in a systems.

Consider system in which  and  phase can form. Let us write chemical equation representing equilibrium between any two condensed phases

.



Since  and  are pure and hence their activities are unity.



We see that  equilibrium is independent of  and hence it is a vertical line AB in diagram 15.1. The actual values of  and  can be obtained from free energy values.
Similarly line DG represents equilibrium between  and



Since activity of condensed phase is unity



DG line is also a vertical line.
Consider  equilibrium

We not that equilibrium between  and  can be attained by varying  and  both and the line DH shows the variation of against.