Chemists use the term precipitate to describe a solid that forms and separates from a liquid solution. Precipitates generally form because the chemical reaction changes one of the chemical's ability to dissolve in solution.
Chemists often use this type of precipitation to remove metals or other substances from a solution. For example, if a chemist wants to remove silver ions from silver nitrate, he adds chlorine ions to the solution. The chlorine ions and silver ions combine to form the insoluble chemical silver chloride. Because silver chloride is not soluble in water, it cannot remain dissolved and separates out as a precipitate.