Mercury(II) oxide, also called mercuric oxide or simply mercury oxide, is the inorganic compound with the formula HgO. It has a red or orange color. Mercury(II) oxide is a solid at room temperature and pressure. The mineral form montroydite is very rarely found. Properties Chemical formula HgO Molar mass 216.591 g·mol?1 Appearance Yellow or red solid Odor odorless Density 11.14 g/cm3 Melting point 500 °C (932 °F; 773 K) (decomposes) Solubility in water 0.0053 g/100 mL (25 °C) 0.0395 g/100 mL (100 °C) Solubility insoluble in alcohol, ether, acetone, ammonia Band gap 2.2 eV[1] Magnetic susceptibility (?) ?44.0·10?6 cm3/mol Refractive index (nD) 2.5 (550 nm)[1] Structure Coordination geometry orthorhombic Thermochemistry Std molar entropy (S?298) 70 J·mol?1·K?1[2] Std enthalpy of formation (?fH?298) ?90 kJ·mol?1[2] Hazards Occupational safety and health (OHS/OSH): Main hazards Extremely toxic, environmental pollutant