Solubility equilibria of sparingly soluble salts

For a sparingly soluble salt in contact with its saturated solution, the product of ion concentrations, each raised to its stoichiometric power, is the solubility product Ksp.

For BaSO₄ ⇌ Ba²⁺ + SO₄²⁻, Ksp = S² where S is the molar solubility.

For a general salt MₓXᵧ, Ksp = xˣ yʸ S^(x+y); for M₂X, Ksp = 4S³, and for MX₂ (like CaF₂), also 4S³.

If the ionic product in a solution exceeds Ksp, precipitation occurs; if it is less, the solution is unsaturated and more solid can dissolve.

A common ion lowers the solubility of a sparingly soluble salt; this is used to purify common salt by passing HCl gas through its saturated solution, which precipitates NaCl.

Salts of weak acids become more soluble at lower pH, because the anion is removed by protonation.

Compare the solubility of salts by comparing S, not Ksp directly, unless they have the same formula type.