Solubility

Solubility is the property of a substance to form a solution in a particular solvent (usually water). It is often expressed in units such as grams per liter (g/L) or molarity (mol/L) and depends on factors such as temperature, pressure, and the type of solvent. For comparison, ordinary rock salt (NaCl) has a solubility in water under standard conditions of 357 g/L, while fluorite ( _CaF₂ ) has a solubility of approximately 0.016 g/L.

The solubility product (K _{_sp}) is also frequently given. This is a dimensionless, temperature-dependent equilibrium constant between the solid and dissolved ions, calculated as the product of the concentrations of the solutes in an equilibrated solution. For example, in the reaction AgCl ↔ Ag ^⁺ + Cl^-, K _{_sp} = [Ag ^⁺][Cl ^{^-} ]. The solubility product indicates the amount of ions that can dissolve before the solution is saturated and the solid and solution reach equilibrium. A high solubility product means that a large amount of solid can dissolve before a saturated solution and thus equilibrium are reached; a low solubility product indicates that the solid is sparingly soluble and that only a small amount of the solid needs to dissolve in water before a saturated solution (at equilibrium) is reached. Therefore, the solubility product also provides information about whether precipitation/crystallization can occur. If the product of the ion concentrations is greater than the solubility product (e.g., due to local enrichment), the solution is supersaturated. This causes the excess solute to precipitate in order to restore equilibrium. Under standard conditions, for example, the solubility product of rock salt (NaCl) is 37.21 and that of fluorite ( _CaF₂ ) is 2.51 × ^10⁻¹¹ .