Sentence:  The liquid phase photochemical exchange between <WSD>chlorine</WSD> and carbon tetrachloride was more reproducible than the thermal exchange , although still erratic . 
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)