solubility product constant

Meaning: Ksp is the equilibrium constant for dissolving a slightly soluble ionic solid into its aqueous ions. The pure solid is left out, and each dissolved ion is raised to the power of its coefficient. Compare the ion product Q (the same expression with the current concentrations) with Ksp. Q < Ksp means the solution is unsaturated, Q = Ksp means it is saturated, and Q > Ksp means a precipitate forms until Q falls to Ksp.

Equation: For CaF₂(s) ⇌ Ca²⁺(aq) + 2F⁻(aq), Ksp = [Ca²⁺][F⁻]², where each bracket is a concentration in mol/L (Ksp is usually quoted without units). If the molar solubility is s mol/L, then Ksp = (s)(2s)² = 4s³.

On the MCAT: Expect a passage that gives Ksp and asks for molar solubility, or that mixes two solutions and asks whether a precipitate forms (calculate Q, then compare). The common-ion effect appears often: adding an ion already in the equilibrium lowers solubility. Salts with basic anions, such as hydroxides and carbonates, dissolve more in acid because H⁺ consumes the anion.

Common confusion: A smaller Ksp means lower solubility only when the salts release the same number of ions, so compare AgCl with AgBr, not AgCl with CaF₂. Adding a common ion lowers solubility, but Ksp itself changes only with temperature.