For the sparingly soluble binary salt, MX, we can represent its dissolution in water as:
MX(s) → M+(aq) + X−(aq).
This is an equilibrium reaction. The (aq) designates the aquated ion, i.e. a metal ion or a negative ion that is aquated or solvated by several water molecules. As for any equilibrium, we can write the equilibrium reaction:
Ksp = [M+(aq)][X−(aq)][MX(s)]
Now both [M+] and [X−] can be measured in that there are measurable concentrations in g⋅L−1 or mol⋅L−1, but we cannot speak of the concentration of a solid; so [MX(s)] is meaningless.
So now (finally!), we have, Ksp = [M+][X−].
This solubility expression (the solubility product!) is dependent solely on temperature (a hot solution can normally hold more solute than a cold one). Ksp constants have been measured for a great number of sparingly soluble salts, and assume standard laboratory conditions.
Because it is a constant, the greater the Ksp, the more soluble the solute. Note that Ksp expressions do not differentiate as to the source of the M+ and X− ions. The salt should be less soluble in a solution where X− ions were already present.
