Historical clue formulations

• Corrosive chemical, to a chemist — 2008-05-11
• Stomach acid, to a chemist — 2008-01-24
• Corrosive chemical, to a chemist — 2006-01-15
• Stomach acid, symbolically — 2002-09-25
• Colorless gas with a pungent odor, for short — 2001-12-16