Gas, Cl (8)
I believe the answer is:
chlorine
I believe this is a double definition.
'gas' is the first definition.
(I know that chlorine is a type of gas)
'cl' is the second definition.
(synonyms)
(Other definitions for chlorine that I've seen before include "Pale green gas" , "It's used to purify water" , "Yellowish-green gas" , "Chemical used as poison gas and water supply disinfectant" , "It's used to purify a swimming pool" .)