The common-ion effect occurs when a solution already contains an ion that is part of a dissolved compound, which suppresses the compound’s ability to dissolve further.
In simple words:
- If you have a solution with a certain ion and you add more of that same ion from another source, the solubility of the compound decreases.
- This happens because the system tries to maintain equilibrium, so the added ion shifts the reaction backward (precipitation is favored).
Example (without equations):
- If a solution contains sodium ions and you add more sodium from another salt, a salt that contains sodium becomes less soluble.
In short: Adding a common ion reduces the solubility of a compound because the system shifts to maintain equilibrium.