Precipitation Reactions occur when cations and anions of aqueous solutions combine to form an insoluble ionic solid, called a precipitate. Whether or not such a reaction occurs can be determined by using the solubility rules for common ionic solids. Since not all aqueous reactions form precipitates, one must consult the solubility rules before determining the state of the products and writing a net ionic equation. Being able to predict these reactions allows scientists to calculate what ions are present in a solution, and allows industries to form chemicals by extracting certain elements from these reactions.