When a solute forms a saturated solution in a solvent, the concentration of the solute, at a given temperature, is determined by the equilibrium constant at that temperature.
The activity of a pure substance in the solid state is one, by definition, so the expression simplifies to
If the solute does not dissociate the summation is replaced by a single term, but if dissociation occurs, as with ionic substances 
For example, with NaSO,  and  so the solubility product is written as
Concentrations, indicated by [...], are usually used in place of activities, but activity must be taken into account of the presence of another salt with no ions in common, the so-called salt effect. When another salt is present that has an ion in common, the common-ion effect comes into play, reducing the solubility of the primary solute.