In a saturated solution, what is the correct description of the solubility product constant Ksp?

• A. Ion product equals Ksp when precipitation has begun but not progressed.
• B. Ion product greater than Ksp means solution is at equilibrium.
• C. Ion product equals Ksp indicates the solution is saturated and at equilibrium.
• D. Ion product cannot be used to infer saturation.

Answer: (C)

For a sparingly soluble salt, the solubility product constant Ksp represents the condition where dissolution and precipitation balance each other. In a saturated solution, the concentrations of the ions are just right so that the ion product—multiplying the ion concentrations in the dissolution stoichiometry—equals Ksp. That equality shows the system is at equilibrium with the solid; the solution cannot dissolve more salt unless conditions change.

If the ion product were smaller than Ksp, the solution would be unsaturated and more solid could dissolve to reach saturation. If the ion product were larger than Ksp, the solution would be supersaturated and precipitate would form until the ion product returns to Ksp. Therefore, the statement that the ion product equals Ksp indicates the solution is saturated and at equilibrium is the correct one.