AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Onion (or turmeric) is an olfactory indicator. Its smell changes in the presence of acids and bases, helping to detect them.
Source: Acids, Bases and Salts, Chapter 2
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The chapter on Acids, Bases and Salts mentions olfactory indicators — substances whose smell changes in acidic or basic conditions (e.g., onion, vanilla). Examiners expect you to name one such indicator. "Onion" is the most commonly cited example in NCERT/CBSE context. Do not confuse olfactory indicators with visual ones like litmus or turmeric.