Solubility equilibrium

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:

<math>CaCO_3(s) \Leftrightarrow Ca^{2+}(aq) + CO_3^{2-}(aq)</math>

The equilibrium constant for this reaction is

<math>K_c = \frac{\left[Ca^{2+}(aq)\right]\left[CO_3^{2-}(aq)\right]}{ \left[CaCO_3(s)\right]}</math>

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):

<math>K_{sp} = \left[Ca^{2+}(aq)\right]\left[CO_3^{2-}(aq)\right]</math>

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.