What does the common ion effect describe?

The common ion effect refers specifically to the phenomenon where the solubility of an ionic compound decreases when a common ion is introduced into the solution. This occurs due to Le Chatelier's principle, which states that if a system at equilibrium is disturbed, the system will adjust to counter that disturbance. When a common ion, which is an ion already present in the solution, is added, the equilibrium shifts toward the left to reduce the concentration of the ions produced by the dissociation of the salt, thus decreasing its solubility.

For instance, if you take a salt like calcium fluoride (CaF2) that dissociates into Ca²⁺ and F⁻ ions in solution, and you add sodium fluoride (NaF), which contributes additional F⁻ ions, the increased concentration of F⁻ ions will push the equilibrium to favor the undissociated form of CaF2, resulting in lower solubility of CaF2.

The other options address different concepts. The first option suggests an increase in solubility, which contradicts the essence of the common ion effect. The third option discusses reaction rates, which does not relate to changes in solubility due to common ions. The fourth option about the influence of p