Homogeneous and heterogeneous equilibria

Imagine that all substances participating in an equilibrium are homogeneous substances, than we call that a homogeneous equilibrium. Every substance that participates has any concentration (in the case of solutions and liquids) or pressure (in the case of gases).

The following equilibriuim is heterogeneous: CaCO3(s) CO2(g) + CaO(s)

Imagine a closed reaction room with a certain amount of Calcium carbonate on the bottom. Then this is well heated until the carbonate starts to decompose. The gas Carbon dioxyde and CaO (dry chalk) is produced.
In the reaction cylinder (under the piston) is a chemical equilibrium between the two solids (heterogeneous on the bottom) and the gas (homogeneousin the room). The only substancewith a concentration or pressure is here the gas Carbon dioxyde.
The solids receive in the equation, in the formula for the equilibrium constant the value 1, as said before.

Another example of a heterogeneous equilibrium is the dissociation of an insoluble salt in water.
The word 'insoluble' is not enough exact, because there always will be some ions escaping from the lattice when the salt is in the water. The escaped ions (the products) react back to reproduce the salt lattice.

The following equilibrium is realised:
CaCO3(s) Ca2+(aq) +CO32-(aq)

Out of the four components in this mixture, one is solvent and three participate in the proper reaction.