Reactive Gas Used in Toothpastes
Puzzle 20 of Valuable Things Hard Pack
Question is: Reactive Gas Used in Toothpastes
and the answer is: FLUORINE
Word Meaning:
FLUORINE — (uncountable) The chemical element (symbol F) with an atomic number of 9. It is the lightest of the halogens, a pale yellow-green, highly reactive gas that attacks all metals.
Definitions from Wiktionary.