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.
