NCERT Solutions Curiosity Chapter 2 Activity 2.6: Let us investigate — — Onion as an olfactory indicator
Book page 16 Updated on2026-09-05
Q1.
Take some finely chopped onions in a container, along with some strips of clean cotton cloth or filter paper. Tightly close the container and leave it overnight. Take two of the cotton cloth or filter paper strips from the container and check their odour.
Answer
After a night in the closed container the strips smell strongly of onion, even though no onion is touching them any more.
Why it happens: the substance responsible for the onion smell escapes into the air of the closed container and settles into the cloth or paper. Keeping the container tightly closed is what makes this work — in an open dish the smell would simply escape into the room and the strips would pick up almost nothing.
Tip: check the odour by wafting the strip towards your nose with your hand rather than pushing it right against your nostrils — the standard laboratory way to smell anything.
Q2.
Keep them on a clean surface and put a few drops of tamarind water on one strip and a few drops of baking soda solution on the other. Allow the drops to spread on the strips. Check the odour again. Do you notice any change in the odour of the onion strips before and after putting tamarind water and baking soda solution on them? Note your observations.
Answer
Yes, there is a clear difference between the two strips.
Strip
Substance added
Nature
Odour after adding
Strip 1
Tamarind water
Acidic
Onion smell still clearly there
Strip 2
Baking soda solution
Basic
Onion smell almost gone
Why it happens: the smelly substance in onion reacts with a base and is changed into something that has no smell. An acid does not do this, so the acidic strip keeps its onion odour. The nose is therefore giving you the same acid/base information that litmus gives your eyes — which is why onion is an olfactory indicator.
Q3.
Similarly, test the change in the odour with other acidic and basic substances and record your observations.
Answer
Repeat with more samples from Activity 2.1 and the pattern holds. Here is a record you can compare yours with.
Substance put on the onion strip
Nature
Change in odour
Lemon juice
Acidic
Onion smell remains
Vinegar
Acidic
Onion smell remains
Soap solution
Basic
Onion smell greatly reduced
Lime water
Basic
Onion smell greatly reduced
Tap water
Neutral
No noticeable change
The conclusion: the onion strip separates basic substances from the rest by smell alone. Like turmeric paper, it cannot tell an acid from a neutral substance — but unlike turmeric paper, it needs no light and no eyesight.
Try This: repeat the whole activity with a few drops of vanilla essence on the strips instead of onion. The result is the same and the laboratory smells much better.