A reaction that forms a precipitate is one in which two different solutions are mixed, thus forming a particular product that is not soluble in the solvent and crashes out, or precipitates, as a solid which can be filtered out of the solvent. One particular precipitation reaction that is used in industry for a medical application is the formation of barium sulfate from barium chloride and sodium sulfate. The reaction is outlined below.
BaCl2 + Na2SO4 --> BaSO4 + 2NaCl
The barium chloride and sodium sulfate both completely dissolve in water, but when the two solutions are mixed, barium sulfate will precipitate out as a solid. Barium sulfate is utilized in the medical imaging industry because it absorbs x-rays effectively and can show up in the body during an imaging scan.