Ionic equilibrii..

Solubility product is applicable only for sparingly soluble salts.

Solubility product at a given temperature is equal to the product of the concentrations of its ions in the saturated solution, with each concentration term raised to the power equal to the number of ions produced on dissociation of one mole of a substance.

There is another product called ionic product which is defined as the product of the concentrations of its ions at any concentration, with each concentration term raised to the power equal to the number of ions produced on dissociation of one mole of a substance.

Now, if the ionic product of a salt is greater than the solublity product, the excess ions gets precipitated.