How does a common ion affect solubility?

Question

anonymous · Answer

the "common ion effect" usually refers to this scenario:

a beaker contains a solution of a salt with low solubility, e.g. AgCl and there is some precipitate present. then this equilibrium applies:
$$AgCl \leftrightarrow  Ag^+ + Cl^-$$
what happens when we add a concentrated solution of a soluble salt that contains one of the ions, such as AgNO3? 

according to the lechatlier principle, the extra Ag^+ will cause the equilibrium to the reactants side. as a result, some Ag^+ will take some of the originally dissolved Cl^- and come out as a precipitate, and we have more precipitate then before.

in one sentence: the common ion effect causes the solubility to decrease, but it's just an example of applying the lechatlier principle to solubility equilibria.