potassium nitrate, chemical compound, KNO3, occurring as colorless, prismatic crystals or as a white powder; it is found pure in nature as the mineral saltpeter, or niter. (The name saltpeter is also applied to sodium nitrate, although less frequently.) It is slightly soluble in cold water and very soluble in hot water. Potassium nitrate is prepared commercially by the reaction of potassium chloride with sodium nitrate. When potassium nitrate decomposes (on heating) it releases oxygen; it has been used extensively as the oxygen-supplying component of gunpowder since about the 12th cent. It is also used in explosives, fireworks, model rocket propellants, matches, and fertilizers, as a preservative in foods (especially meats), and in the manufacture of nitric acid and of glass.