Sentence:  When typical reaction cells to which 0.3 of an atmosphere of oxygen had been added were illuminated , <WSD>chlorine</WSD> and phosgene were produced . 
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)