Which one of the following can be used as an acid-base indicator by a visually impaired student?
(c) Vanilla essence
(d) Petunia leaves
Vanilla essence is an olfactory indicator and it has a characteristic pleasant odour. If a basic solution is added to vanilla extract then we cannot detect the characteristic odour of vanilla extract. On the other hand, an acidic solution, however, does not destroy the odour of vanilla extract. So, an olfactory indicator exhibiting such behaviour (different odours) in different medium can be used by a visually impaired student. Hence, option (c) is the correct answer.