Tungsten is not oxidised appreciably in air below red heat, nor is it at any known temperature, unless in the vaporous state, attacked by nitrogen. Halides are formed directly, the fluoride at ordinary temperatures, the iodide at a bright red heat, and the others intermediately. On heating in the electric furnace with carbon, boron, or silicon, combination takes place. Neither sulphur nor phosphorus has any action upon the metal. At a red heat it decomposes water; it is not attacked by a solution of caustic potash, but by fused alkalis it is dissolved, though more rapidly by a fused mixture of potassium carbonate and nitrate. Oxidation attended by incandescence takes place on heating the powdered metal with certain oxidising agents such as lead dioxide and potassium chlorate.
Tungsten, like molybdenum, is very resistant to the action of acids; this is largely due to the formation of a protective coating of oxide. Neither aqua regia nor hydrofluoric acid dissolves the metal to any appreciable extent: the best solvent for the fused metal is a mixture of concentrated nitric and hydrofluoric acids. |