Consider an ionic solid that dissolves in water according to the equation:
The equilibrium constant for the reaction,
is defined as the solubility product of MnXm. The form of this equilibrium is important in understanding effects such as the influence of complexion and pH and also the common ion effect: it may be seen that adding one of the ions Mm+ or Xn- will shift Reaction 1 to the left and so less the solubility of the salt. Thus AgCl(s) is much less soluble in a solution containing 1 M Ag+ (e.g. from soluble AgNO3) than otherwise.