Precipitates

Precipitates are solid materials formed when dissolved ions in a solution react to form an insoluble compound. The solid that forms is called a precipitate, and the reaction is a precipitation reaction. Precipitation occurs when the product of the concentrations of the ions exceeds the solubility product (Ksp) of the solid; the ions combine to form a solid that separates from the solution.

Formation involves nucleation (formation of initial tiny particles) followed by growth of crystals. Nucleation can be homogeneous or heterogeneous (on surfaces, impurities). Conditions such as concentration, temperature, pH, and the presence of complexing agents influence whether precipitation occurs and the morphology of the solid. Supersaturation drives rapid precipitation and can yield fine powders or amorphous solids; slower conditions promote larger crystals.

Common inorganic precipitates include silver chloride (AgCl) formed from Ag+ and Cl-, barium sulfate (BaSO4), calcium carbonate (CaCO3). Precipitation reactions are used in qualitative inorganic analysis and gravimetric analysis, as well as in water treatment to remove hardness, sulfates, heavy metals. In geology and biology, minerals such as calcite or gypsum can precipitate from aqueous solutions.

Precipitation is distinct from crystallization, though both can produce solids from a solution. Crystallization emphasizes orderly lattice formation from supersaturated solutions, while precipitation can yield amorphous or crystalline solids and may be rapid.

In analytical chemistry, selective precipitation and sequential precipitation exploit differences in solubility to separate ions.