Which one of the following can be used as an acid-base indicator by a visually impaired (blind) student ?
Generated by claude-sonnet-4-6 · 2026-06-15 06:42 · grounding rag
Model Answer
Answer: B — Vanilla essence
Vanilla essence is an olfactory indicator — its odour changes in acidic/basic media, making it suitable for visually impaired students who cannot detect colour changes.
Explanation
- Visually impaired students cannot observe colour changes, so visual indicators (turmeric, methyl orange, litmus) are not useful for them.
- Olfactory indicators work by change in smell, not colour. The textbook (Activity 2.2) identifies vanilla essence as one such olfactory indicator — its characteristic odour is lost in a basic (NaOH) solution but retained in an acidic one.
- Onion and clove oil are also olfactory indicators mentioned in the chapter, but vanilla essence is the one given in the options.