Phosphorus heptasulfide, free from yellow and white phosphorus appears as light-yellow crystals, light-gray powder, or fused solid. Melting point 310°C. Density 2.19 g / cm3. May spontaneously heat and ignite under exposure to moisture. Produces phosphorus pentaoxide and sulfur dioxide during combustion. Phosphorus pentaoxide reacts with water to form phosphoric acid, a corrosive material. Phosphorus heptasulfide reacts with water to form phosphoric acid and hydrogen sulfide, a toxic flammable gas with a rotten-egg smell. Continued exposure to hydrogen sulfide disables the sense of smell.