Sentence:  This <WSD>chlorine</WSD> carbon tetrachloride solution was illuminated for a day following which the flask was resealed onto a vacuum system and the excess chlorine distilled off . 
Meaning of the word marked with <WSD> tags:  a common nonmetallic element belonging to the halogens; best known as a heavy yellow irritating toxic gas; used to purify water and as a bleaching agent and disinfectant; occurs naturally only as a salt (as in sea water)