Chemical formula VO2. Molecular weight 82.94. Dark blue powdery crystals. Specific gravity 4.339. Melting point 1967 ℃. Flammable. It is in the form of sodium chloride. Insoluble in water, soluble in acid and alkali. Amphoteric: soluble in acid to generate tetravalent vanadyl ion (VO2+); soluble in alkali to generate hypovanadate (M2V4O9 and M2V2O5). Chemically unstable. It is slowly oxidized in the air and reacts with nitric acid to form vanadium pentoxide. It is prepared by heating vanadium pentoxide and vanadium trioxide in equimolar ratio isolated from air or heating and melting vanadium pentoxide with oxalic acid, carbon, phosphorus and sulfur dioxide. Vanadium dioxide is mainly used as a high temperature catalyst.