What does the common ion effect describe?

The common ion effect describes the phenomenon where the solubility of a sparingly soluble ionic compound decreases in a solution that already contains a common ion. When a common ion is introduced into the solution, it causes the equilibrium of the dissolution reaction to shift, according to Le Chatelier's principle. This shift results in reduced solubility of the ionic compound because the presence of the common ion increases the concentration of that particular ion in solution.

For example, consider the dissolution of barium sulfate (BaSO₄). If barium ions (Ba²⁺) are added to a solution already containing BaSO₄, the added Ba²⁺ ions will shift the dissolution equilibrium, leading to less BaSO₄ dissolving in the solution than would occur without the added common ion. This effect is significant in various chemical contexts, especially in precipitate formation and separating mixtures.

The other options do not accurately describe the common ion effect. An increase in solubility with more ions added, complete dissolution, or changes due to temperature would pertain to different aspects of solubility and do not address the specific response of solubility to the presence of a common ion.