Which statement correctly describes the common ion effect?

• A. Adding a salt containing one of the ions of a sparingly soluble salt reduces its solubility.
• B. Adding a salt increases the solubility of all salts.
• C. Common ion effect means pH changes do not affect solubility.
• D. It only applies to gases.

Answer: (A)

The common ion effect describes how the solubility of a sparingly soluble salt decreases when a salt containing one of its ions is added to the solution. This happens through Le Chatelier’s principle and the dissolution equilibrium: adding a common ion increases its concentration, so the system shifts toward forming more solid and dissolving less of the salt. Since the solubility product Ksp must remain constant, the extra common ion pushes the equilibrium left, lowering the amount of the salt that can dissolve. That is exactly what the statement captures: introducing a salt with a common ion reduces solubility.

The other ideas aren’t generally true: adding a salt does not universally increase solubility for all salts, and pH changes can affect solubility for many salts, especially those involving acidic or basic ions or complexation. The common ion effect also isn’t restricted to gases; it applies to solids dissolving in solution as well.