If the reaction quotient Q for a sparingly soluble salt is greater than Ksp, what happens?

• A. Dissolution proceeds until Q falls below Ksp.
• B. No change; system remains oversaturated.
• C. Dissolution continues beyond saturation.
• D. Precipitation occurs until Q equals Ksp.

Answer: (D)

When a sparingly soluble salt is in water, its dissolved ions and the solid form are related by Ksp, the solubility product. The reaction quotient Q uses the actual ion concentrations right now. If Q is greater than Ksp, the solution is supersaturated with respect to that salt, so the system moves toward removing ions from solution by forming more solid. This precipitation continues until the ion activities drop enough that Q falls to equal Ksp, at which point the solution is at equilibrium and no net change occurs. So the correct description is that precipitation occurs until Q equals Ksp. The other possibilities would require adding ions or doing nothing, which would not restore equilibrium when the solution is supersaturated.