Boron is black or dark brown powder, which is oxidized in air and hinders the oxidation of boron due to the formation of three oxidation two boron film. It can react with fluorine at room temperature without corrosion of hydrochloric acid and hydrofluoric acid aqueous solution. Boron is insoluble in water, and powder like boron can dissolve in boiling nitric acid and sulfuric acid, and most molten metals such as boron, copper, iron, manganese, aluminum and calcium. Melting point 2076. Boiling point 3927. Boron is a weak conductor at normal temperature, and it conducts well at high temperature. There are many allotropes of elemental boron, amorphous boron powder is brown, grayish black crystal boron. The hardness of boron is similar to that of diamond, and it has high resistance, but its conductivity increases with the increase of temperature. Crystalline boron is inert and amorphous boron is more active. There are 14 isotopes in boron, only two of them are stable.