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.

In addition, the solubility product (K _{_sp}) is often specified. 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 K_sp quantifies the thermodynamic equilibrium between a solid phase and its dissolved components in a saturated solution. A high K_{_sp} value characterizes readily soluble phases where high ionic activities exist at equilibrium. Conversely, a low value indicates a sparingly soluble phase where even small amounts of the substance lead to saturation. The solubility product thus serves as an indicator of potential precipitation and crystallization processes. If the current ion product (Q) exceeds the solubility product (K_{_sp} ), for example due to local concentrations or evaporation, a state of supersaturation exists. To restore thermodynamic equilibrium, the supersaturated phase precipitates/crystallizes. Under standard conditions, the solubility product of rock salt (NaCl) is, for example, 37.21 and that of fluorite (CaF_₂) is 3.5 × 10 ^{^-11} . Therefore, fluorite is significantly less soluble than rock salt.