The heterogeneous equilibrium

A vapour in contact with the pure solid (or liquid)

Experiment

Two test tubes contain different quantities of iodine . We have a solid phase where iodine is pure and a gas phase. The system is heterogeneous. The two tubes are put at the same temperature:

The intensity of coloration in the gas phase (therefore the partial pressure of iodine) seams to be the same.

Interpretation

As iodine is alone in the lower solid phase, its concentration $c=[I_2(s)]$ is constant therein. We apply the law of mass action: $\frac{[I_2(g)]}{[I_2(s)]}$ $=$ $K$ $\frac{[I_2(g)]}{c}$ $=$ $K$ $[I_2(g)]$ = $K\cdot c$ Using the ideal gas law, let us examine the partial pressures of iodine in the gas phase : $P_{I_2(g)}$ $=$ $[I_2(g)]\cdot R\cdot T$ $P_{I_2(g)}$ $=$ $K \cdot c \cdot R\cdot T$ A new constant is found as a product of four constants: $P_{I_2(g)}$ $=$ $K_{sat}$

Generalization

When a substance $X$ is in equilibrium between gaseous phase and a pure liquid or solid phase , and when it stays pure in the last phase, its partial pressure in the gaseous phase is constant at a given temperature when equilibrium is attained: $P_X(g)$ $=$ $K_{sat}$ $K_{sat}$ is a constant at a given temperature called saturation vapour pressure of substance X in the gaseous phase.