MoO₃; white at ordinary temperatures; yellow at elevated temperatures; molecular weight, 143.94; orthorhombic; sp gr, 4.69 (at 21 degrees C); melting point, 795 degrees C; boiling point, 1,264 degrees C or sublimes at 1,155 degrees C, at 1 atmophere (101 kPa); sparingly soluble in water; very soluble in excess alkali with the formation of molybdates; and soluble in concentrated mixtures of nitric and hydrochloric acids and in mixtures of nitric and sulfuric acids.