phosphorus pentoxide

noun

: a compound known in various polymeric forms (P₂O₅)_x that is obtained usually by burning phosphorus in an excess of dry air, occurs as a white powder that reacts vigorously and sometimes explosively with water to form phosphoric acids irritating to the skin and mucous membranes, and is used chiefly as a drying agent, as a condensing agent in organic synthesis, and in making phosphoric acids and derivatives