In chemistry, precipitation is the process of transforming a dissolved substance into an insoluble solid from a supersaturated solution). The solid formed is called the precipitate. Precipitation reactions occur when cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate/Descriptive_Chemistry/Main_Group_Reactions/Reactions_in_Aqueous_Solutions/Precipitation_Reactions). The use of solubility rules can determine whether or not such a reaction occurs/Descriptive_Chemistry/Main_Group_Reactions/Reactions_in_Aqueous_Solutions/Precipitation_Reactions). Most precipitation reactions are single replacement reactions or double replacement reactions/Descriptive_Chemistry/Main_Group_Reactions/Reactions_in_Aqueous_Solutions/Precipitation_Reactions). Precipitation reactions are useful in determining whether a certain element is present in a solution/Descriptive_Chemistry/Main_Group_Reactions/Reactions_in_Aqueous_Solutions/Precipitation_Reactions). Chemical precipitation is the most common technology used in removing dissolved (ionic) metals from solutions, such as process wastewaters containing toxic metals.

An important stage of the precipitation process is the onset of nucleation. The creation of a solid particle implies the formation of an interface with the solution. This involves energy changes depending on the dissolution reaction free energy (endothermic or exothermic process accompanied by an entropy increase) and the relative surface energy developed between the solid and the solution). While precipitation reactions can be used for making pigments, removing ions from solution in water treatment, and in classical qualitative inorganic analysis, precipitation is also commonly used to isolate the products of an organic reaction during workup and purification operations).

In summary, precipitation in chemistry is the process of transforming a dissolved substance into an insoluble solid from a supersaturated solution. The solid formed is called the precipitate. Precipitation reactions occur when cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate. The use of solubility rules can determine whether or not such a reaction occurs. Chemical precipitation is the most common technology used in removing dissolved (ionic) metals from solutions.