Olfactory indicator
- CBSE · NCERT Class 10, Ch 2 Acids, Bases and Salts
An olfactory indicator is a substance whose smell changes in an acid or a base, so you can test a solution by smelling it.
Olfactory means to do with smell. Most indicators change colour, like litmus. These change smell instead. NCERT names three to try: onion, vanilla essence and clove oil.
The test is simple. Add a few drops of the indicator to the solution, shake it, and check the smell. Whether the smell stays or goes tells you acid or base.
A small example
Keep 2 strips of cloth with chopped onion in a closed bag overnight. Both strips now smell of onion. Put a few drops of dilute hydrochloric acid on one, and dilute sodium hydroxide on the other. Rinse and smell again. The strip with the acid still smells of onion. The strip with the base has lost the smell. So the smell tells acid from base.
Where marks go
Question 24 of CBSE's 2026-27 Science sample paper is an assertion-reason question. The assertion: onion loses its smell in baking soda solution but keeps it in lemon juice. The reason: onion is an olfactory indicator.
The 2026-27 Science marking scheme marks option B: both statements are true, but the reason is not the correct explanation. So judge each statement on its own first. Then ask the separate question: does the reason explain the assertion? It is worth 1 mark, with no part credit.
Sources
Facts last checked against these sources on 30 September 2026.
About the author
Founder Member & Academic Lead
An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.
Deepti Sardana on LinkedIn
