If the ion product Q for the dissolution of a sparingly soluble salt exceeds its solubility product Ksp, which statement best describes the system?

• A. No change
• B. Precipitation occurs immediately
• C. The rate of dissolution increases
• D. Precipitation occurs until equilibrium is reached

Answer: (D)

When a sparingly soluble salt dissolves, the solubility product Ksp sets the maximum product of the ion concentrations at equilibrium. The ion product Q is the actual [cation][anion in solution]. If Q exceeds Ksp, the solution is supersaturated with respect to the solid, so the system shifts toward forming more solid to reduce the ion concentrations until Q drops to Ksp. Thus precipitation occurs until equilibrium is reached, at which point the ionic concentrations satisfy Ksp and solid and dissolved ions can coexist.

No change would occur only if Q were equal to Ksp. The rate of dissolution would not increase because dissolution is favored when Q < Ksp. Precipitation does not stop immediately but continues until the system re-establishes equilibrium.