Solubility Equilibrium

When you try to dissolve a solid into water, if it dissolves, there are three possibilities:

  1. The solid is a non-electrolyte and it dissolves as neutral molecules.
  2. The solid is a highly soluble electrolyte; it dissolves almost completely.
  3. The solid is a sparingly soluble electrolyte; it dissolves to a limited extent.

When silver chloride is added to water, the following equilibrium is established:

AgCl (s) ⇌ Ag+ (aq) + Cl- (aq)

This is an example of a heterogeneous equilibrium because it involves both a solid and a solution. This equilibrium is known as the solubility equilibrium. As a matter of convention the concentration of the undissolved solid is taken as one.

Ksp = [Ag+] [Cl-]

The equilibrium constant is the product of the concentrations of the ions. It is called solubility product constant. The solubility product constant of a given salt is constant at a given temperature.