Which one of the following can be used as an acid-base indicator by a visually impaired student?
(a) Litmus
(b) Turmeric
(c) Vanilla essence
(d) Petunia leaves
Answer: (c)
Vanilla essence is an olfactory indicator. So, its smell is different in acidic and basic media, which can be detected easily by a visually impaired student. If a basic solution is added to vanilla extract, then we cannot detect the characteristic smell of vanilla extract. An acidic solution does not destroy the smell of vanilla extract.