25.1.7—Terms: solubility product, K sp

The solubility product Ksp is the equilibrium constant for a solid dissolving into its aqueous ions. Pure solids are omitted; aqueous-ion concentrations are raised to their stoichiometric powers.

A small Ksp generally indicates low solubility, but the numerical comparison is meaningful only for salts with comparable dissolution stoichiometry.

For AgCl(s) ⇌ Ag⁺ + Cl⁻, Ksp=[Ag⁺][Cl⁻]. For CaF₂(s) ⇌ Ca²⁺ + 2F⁻, the expression has [F⁻]².

Ksp is not the concentration of the solid and cannot be compared blindly across different ion powers.