18.4 The Common-Ion Effect

Chemical Concept Demonstrated: Common-ion effect


AgNO3 and NaOAc are added to a beaker of water, and the solution is filtered.
  • The crystallizing dishes are half-filled with the filtrate.
  • AgNO3 is added to the first dish.
  • NaOAc is added to the second.


    AgOAc precipitates in both dishes.

Explanation (including important chemical equations):

    The common-ion effect is the decrease in the solubility of a salt that occurs when the salt is dissolved in a solution that contains another source of one of its ions.  The filtrate is a saturated solution of Ag+ and OAc- ions.  When either AgNO3 or NaOAc is added to the filtrate, the solubility of the ions in the saturated solution decreases, and the Ag + and OAc- ions crash out as AgOAc.

    AgNO3 (aq) + OAc - (aq) <=> AgOAc (s)

    NaOAc (aq) + Ag + (aq) <=> AgOAc (s)