Unary Phase Diagrams

Phase diagrams of a single component system are called as unary diagrams.

As defined unary diagrams are for the single component system, and hence, there is no composition variable. The only two variables are temperature and pressure.

Unary diagram of H₂O :

Applying Gibbs phase rule in single phase region, i.e., C = 1, P = 1 => F = 1 - 1 + 2 => F = 2, there are two degrees of freedom. Means, temperature and pressure can be varied independently. Hence, single phase equilibrium is characterised by an area.

When two phase are in equlibrium, C = 1, P = 2 => F = 1 - 2 + 2 => F = 1, that means, either pressure or temperature can be varied independently. Both can not be varied simultaneously. Hence, two phase equilibrium is possible only along phase boundaries.

At triple point, all three phases are in equilibrium so gibbs rule can be applied as -
As F = C - P + 2
=> F = 1 - 3 + 2
=> F = 0
Means, system does not have any independent variable, Hence, Equilibrium of three phases holds only at a point.

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Unary diagram of pure iron :

A typical unary phase diagram of pure iron is shown below. Here, the solid phases are normally designated by Greek alphabets like \( \alpha , \gamma, \textrm{and } \delta \).
Applying Gibbs phase rule in single phase region, i.e., C = 1, P = 1 => F = 1 - 1 + 2 => F = 2, there are two degrees of freedom. Means, temperature and pressure can be varied independently. Hence, single phase equilibrium is characterised by an area.

When two phase are in equlibrium, C = 1, P = 2 => F = 1 - 2 + 2 => F = 1, that means, either pressure or temperature can be varied independently. Both can not be varied simultaneously. Hence, two phase equilibrium is possible only along phase boundaries.

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