The common ion effect is an effect that suppresses the ionization of an electrolyte when another electrolyte (which contains an ion that is also present in the first electrolyte, i.e. a common ion) is added. It is considered to be a consequence of Le Chatlier’s principle (or the Equilibrium Law) . The common ion effect describes an ions effect on the solubility equilibrium of a substance. If a soluble compound consisting of a common ion is added, it can decrease the concentration of that ion within the solution, which can result in a change in the equilibrium point of the solution. The common ion effect can be used to calculate concentrations involving common ions and ion concentrations involving chemical equilibrium/Equilibria/Solubilty/Common_Ion_Effect).

Examples of the common ion effect include:

• When (\ce{AgCl}) is dissolved into a solution already containing (\ce{NaCl}), the (\ce{Cl-}) ions come from the ionization of both (\ce{AgCl}) and (\ce{NaCl}), and thus, (\ce{/Equilibria/Solubilty/Common_Ion_Effect).
• Gaseous hydrogen chloride passed through a sodium chloride solution leads to the precipitation of the NaCl due to the excess of chloride ions in the solution (brought on by the dissociation of HCl) .
• The solubility of a salt in a solution is affected by the addition of a common ion. For example, adding sodium carbonate to water containing chalk or limestone can decrease the hardness of the water.
• Adding a common ion to a weak acid or weak base prevents it from ionizing as much as it would without the added common ion.

Overall, the common ion effect is an important concept in chemistry that helps explain how the addition of a common ion can affect the solubility of a substance and the ionization of an electrolyte.