Ionic compounds normally dissociate into their constituent ions when they dissolve in water. For example, for silver chloride:
The expression for the equilibrium constant for this reaction is:
where  is the thermodynamic equilibrium constant and braces indicate activity. The activity of a pure solid is, by definition, equal to one.
When the solubility of the salt is very low the activity coefficients of the ions in solution are nearly equal to one. By setting them to be actually equal to one this expression reduces to the solubility product expression:
For 2:2 and 3:3 salts, such as CaSO and FePO, the general expression for the solubility product is the same as for a 1:1 electrolyte
: (electrical charges are omitted in general expressions, for simplicity of notation)
With an unsymmetrical salt like Ca(OH) the solubility expression is given by
Since the concentration of hydroxide ions is twice the concentration of calcium ions this reduces to 
In general, with the chemical equilibrium
and the following table, showing the relationship between the solubility of a compound and the value of its solubility product, can be derived.
Solubility products are often expressed in logarithmic form. Thus, for calcium sulfate, with , . The smaller the value of K, or the more negative the log value, the lower the solubility.
Some salts are not fully dissociated in solution. Examples include MgSO, famously discovered by Manfred Eigen to be present in seawater as both an inner sphere complex and an outer sphere complex. The solubility of such salts is calculated by the method outlined in dissolution with reaction.