If the reaction quotient Q equals Ksp, what is the state of the system?

• A. Precipitation will occur
• B. The solution is supersaturated
• C. The solution is at equilibrium with the solid phase
• D. No ions are present

Answer: (C)

The main idea is that the solubility product Ksp sets the balance point for a sparingly soluble solid dissolving in water. When the reaction quotient Q equals Ksp, the ion product [M⁺][X⁻] in the solution matches exactly the solubility product, so the solution is saturated and the solid is in dynamic equilibrium with the dissolved ions. Dissolution and precipitation happen at equal rates, giving no net change in the amount of dissolved species. There are ions present in the solution, and the solid phase remains in contact with it. If Q were less than Ksp, more solid would dissolve; if Q were greater, ions would precipitate until Q returns to Ksp. So the system is at equilibrium with the solid phase.