All of the equilibria we've looked at so far can be described as homogeneous equilibria. This means that all of the reactants and products are in the same physical state. Often they are all gases or are all in aqueous solution.

Some equilibrium systems are described as heterogeneous equilibria. This means that there are two or more different physical states present.

An example of a heterogeneous equilibrium is the one formed when calcium carbonate is heated in a closed vessel:

CaCO₃(s) _∏ CaO(s) + CO₂(g)

Here we have both solids and a gas present at equilibrium.

Experiments show us that the concentrations of the solids are so very much greater than the concentration of a gas that the concentration of the solids can be regarded as constant. This means that we don't need to include them in the expression for K_c. Instead we can write down a modified version of the equilibrium constant as

K _c = [CO₂]

[INSERT CI_05_ima]

The same idea can be applied to the equilibrium between liquid water and its saturated vapour in a closed vessel.

H₂O(l) _∏ H₂O(g)

Water as a liquid has a much higher concentration than as a vapour and it can be regarded as constant. We can write a modified expression for K_c that doesn't involve the liquid water:

K _c = [H₂O(g)]

These ideas are applied when we have other solids or liquids in heterogeneous equilibria. The concentration of pure solids or pure liquids are constant and are not included in the expressions for K_c.

What would be the expression of K_c for the equilibrium Ag⁺(aq) + Fe²⁺(aq) _∏ Ag(s) + Fe³⁺(aq)?