law of mass action
The law of mass action can be applied to the equilibrium between undissociated molecules of solid in solution and the ions in solution. So if the molecule AB dissociates to give A+ and B- ions, according to the equation:
AB = A+ + B-
the equilibrium constant for this reaction is:
K = ([A+] × [B-])/[AB]
But as the concentration of undissociated solid is constant in a saturated solution, this equation may be rearranged:
[A+] × [B-] = K[AB] = Ks
This new constant Ks is the solubility product. So if the concentration of a univalent anion (e.g., chloride) is doubled in a solution of silver chloride, the concentration of the silver ions is halved.