Solubility equilibrium

Solubility equilibrium is the chemical equilibrium of dissolved and undissolved forms of a substance.

Substances can have varying degrees of solubility. Ionic compounds usually dissolve in water easily whereas gases such as oxygen dissolve much less so. In a closed system, the dissolved and undissolved forms exist in equilibrium. For example, when calcium carbonate dissolves in water the following equilibrium occurs:

The equilibrium constant for this reaction is

For a sparingly soluble solid the solution becomes saturated at a certain point; addition of more calcium carbonate does not result in it being dissolved. The solubility product describes the conditions for equilibrium between a sparingly soluble solid and its saturated solution. For calcium carbonate, omitting the solid form from the equilibrium equation gives its solubility product (K_sp):

At constant temperature and pressure, both the equilibrium constant and the solubility product are constants, which enables the prediction of whether a certain mass of substance will dissolve completely.