Which statement correctly describes the solubility product constant Ksp and precipitation prediction?

• A. Ksp is the product of the activities, not concentrations.
• B. Ksp equals [cation][anion] for sparingly soluble salts; precipitation occurs when Q > Ksp; dissolution when Q < Ksp.
• C. Ksp depends on temperature and changes.
• D. Ksp equals the product of the cation and anion concentrations.

Answer: (B)

The essential idea is that the solubility product constant Ksp sets the threshold for whether dissolved ions stay in solution or start to form a solid. For a sparingly soluble salt, the equilibrium condition is the product of the activities of the cation and the anion in solution: a_C+ × a_A− = Ksp. In dilute solutions, activities are well approximated by concentrations, so Ksp ≈ [Cation][Anion]. The practical rule for precipitation is based on the ion product Q, formed from the current ion concentrations in the solution. If Q exceeds Ksp, the solution contains more ions than can be balanced by the solid, so precipitation occurs until Q drops back to Ksp. If Q is less than Ksp, the solid can dissolve, increasing ion concentrations until Q reaches Ksp. While Ksp itself depends on temperature, the general comparison of Q to Ksp at a fixed temperature is the standard way to predict precipitation versus dissolution.