Q = Ksp indicates ...

• A. A supersaturated solution.
• B. The solution is at equilibrium, saturated.
• C. The solution will dissolve all solid.
• D. No solid can dissolve.

Answer: (B)

For a sparingly soluble salt dissolving in water, the dissolution reaction AB(s) ⇌ A⁺(aq) + B⁻(aq) has a solubility product Ksp = [A⁺][B⁻] at equilibrium. The reaction quotient Q uses the current ion concentrations: Q = [A⁺][B⁻]. When Q equals Ksp, the system is at equilibrium: the rates of dissolution and precipitation are equal, there is solid present, and the solution is saturated. If Q is smaller than Ksp, more solid can dissolve to raise Q to Ksp; if Q is larger than Ksp, the solution will precipitate until Q drops back to Ksp. Therefore, Q = Ksp indicates equilibrium with a saturated solution. The other scenarios correspond to Q ≠ Ksp: supersaturated (Q > Ksp), or conditions where dissolution of all solid or no dissolution would occur in ways not consistent with Q = Ksp.