It is called sulfuric anhydride, a colorless transparent liquid state at room temperature; and when cooled, it becomes colorless ice with smoke. In addition, if it is maintained at 32.5 ° C or higher even in a sealed tube, it becomes a colloidal a-type. Commercially available products are mixtures of type a and b, both of which are powerful oxidizers. It reacts violently with water to make sulfuric acid, and with metal oxide to make sulfate. It is made in large quantity for sulfuric acid, but it is melt in water to become sulfuric acid, and usually made by heating fuming sulfuric acid. For commercial application, it is used as a raw material for cutting-edge aramid fiber, whose marketability has been rapidly increasing.