__ Test, Paper Proof of Acids or Bases
Puzzle 11 of The 1950s Hard Pack
Question is: __ Test, Paper Proof of Acids or Bases
and the answer is: LITMUS
Word Meaning:
LITMUS — (uncountable) A dyestuff extracted from certain lichens, that changes color when exposed to pH levels greater than or less than certain critical levels.
Definitions from Wiktionary.