A test chemical (4)

I believe the answer is:

acid

I believe this is a double definition.

'a test' is the first definition.
(thesaurus)

'chemical' is the second definition.
(thesaurus)

(Other definitions for acid that I've seen before include "Tart - keen - sort of house party" , "Morose" , "Sour - vitriolic" , "Caustic and biting" , "Sour - type of rain or drop" .)

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