Chemistry test paper (6)
I believe the answer is:
litmus
This might be a double definition.
'chemistry' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'test paper' is the second definition.
(litmus is a kind of test paper)
Can you help me to learn more?
(Other definitions for litmus that I've seen before include "'It turns red with acid, blue with alkaline (6)'" , "Old dance" , "paper for chemistry test" , "Substance turned red by acids" , "Substance used to tell acid from alkali" .)