Boron has the largest volumetric heat, and its weight is second only to helium. Slightly soluble in nitric acid, insoluble in water. Mr10.81. ρ = 2.37 g/cm 3 (amorphous); 2.34 to 2.46 g/cm 3 (crystal). The melting point is 2177 to 2301 ° C; the boiling point is 2550 to 2658 ° C.
The boron is a black or dark brown powder which, when oxidized in air, prevents the internal boron from continuing to oxidize due to the formation of the boron trioxide film. It reacts with fluorine at normal temperature and is not corroded by aqueous hydrochloric acid and hydrofluoric acid solutions. Ultrafine amorphous boron powder is insoluble in water, and powdered boron is soluble in boiling nitric acid and sulfuric acid, as well as most molten metals such as copper, iron, manganese, aluminum and calcium.
Ultrafine amorphous boron powder is chemically active. Powder and air can form an explosive mixture. Used in solid stamping engines and smokeless clean propellants.