Polyphosphoric acid is an inorganic polymer compound formed by the dehydration and polymerization of phosphoric acid (H₃PO₄). It is a colorless to pale yellow, viscous liquid (it appears as a glassy solid when the degree of polymerization is high), with no obvious odor, and has strong corrosiveness and hygroscopicity. Its acidity is stronger than that of ordinary phosphoric acid; it is non-volatile, non-flammable, and chemically stable, and can be stored for a long time at room temperature. When heated to high temperatures, it will gradually decompose into phosphoric acid and pyrophosphoric acid.