In a saturated solution of sparingly soluble electrolyte two equilibria exist and can be represented as, ⇌ ⇌
Applying the law of mass action,
Since the solution is saturated, the concentration of unionised molecules of the electrolyte is constant at a particular temperature, i.e., constant.
is termed as the solubility product. It is defined as the product of the concentration of ions in a saturated solution of an electrolyte at a given temperature.
Consider, in general, the electrolyte of the type which dissociates as, ⇌
Applying law of mass action,
When the solution is saturated, (constant) or
Thus, solubility product is defined as the product of concentrations of the ions raised to a power equal to the number of times the ions occur in the equation representing the dissociation of the electrolyte at a given temperature when the solution is saturated.