A solid precipitate can be produced from the combination of two aqueous, ionic solutions. Whether or not the two solutions form a precipitate for certain can be predicted using a series of rules for the solubility of inorganic compounds. The ions in the two compounds interact with each other, but they must produce a compound that is solid and insoluble for it to be considered a precipitate.
The type of reaction that usually produces a precipitate is called a double replacement reaction. Two solutions are required, dissolved in water and containing positively and negatively charged ions, known as cations and anions. By working out the equation of the solutions and their products, the solubility rules can be applied to predict whether or not a particular product that forms is solid.
For example, if a solution of magnesium bromide is combined with a solution of silver nitrate, it produces silver bromide and a magnesium nitrate. The rules stipulate that only three salts containing silver can be dissolved in water: silver nitrate, silver acetate and silver sulfate. Since silver bromide is not one of these, it is expected to sink to the bottom of the liquid magnesium nitrate as a solid precipitate. This prediction can be confirmed by carrying out the reaction in an experiment.
A more commonly known example of a solid product is soap scum. Minerals in hard water combine with soap to produce the precipitate soap scum and an aqueous product.