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NCERT Solutions Curiosity Chapter 2 In-text Questions — — What litmus is, and the Holistic Lens box

Book page 10 Updated on2026-09-05

Q1.
I am curious to know — what are these red and blue litmus paper strips made of? Why do they change colour when drops of some samples are put on them?
Answer

Litmus paper is ordinary paper soaked in litmus, a natural dye obtained from lichens. Two versions are sold — the dye in its blue form and the dye in its red form.

The colour changes because litmus is an acid-base indicator: the dye has one colour when it sits in an acidic liquid and a different colour when it sits in a basic liquid. Put a drop of acid on the blue strip and the dye switches to red; put a drop of base on the red strip and it switches to blue.

Why it happens: the colour of a dye depends on the exact form its molecules are in. An acid pushes the litmus molecule into its red form; a base pushes it into its blue form. Nothing is added to the paper — the same dye is simply being flipped between two forms, which is why the change is instant.
Did you know? Litmus is not the only such dye. The chapter goes on to use red rose petals and turmeric, and beetroot, purple cabbage, jamun and gudhal flowers work too.
Q2.
The substances in Group C, such as tap water, sugar solution, and salt solution, did not change the colour of either litmus paper. Can you predict their nature?
Answer

They are neutral — neither acidic nor basic.

Why it happens: only an acid can flip blue litmus to red, and only a base can flip red litmus to blue. A liquid that leaves both papers untouched contains neither, so there is nothing in it for the dye to respond to. The chapter names this third kind of substance neutral.
Tip: "no change" is a real result, not a failed test. In science an absence of change is often the answer itself.
Q3.
Lichens are formed by the association of two living organisms, a fungus and an alga. They grow on rocks and trees in regions that have abundant rainfall and clean air. Do you find lichens on trees in your neighbourhood?
Answer

Look on the shaded side of old tree trunks, on compound walls and on stone surfaces after the monsoon. Lichens look like flat, crusty or leafy patches — dull green, grey-green, yellow or orange — that seem painted onto the bark.

Sample answer: "In our locality I found grey-green crusty patches on the trunk of an old mango tree and on the boundary wall of the school, but none on the trees along the main road."

Why the answer differs from place to place: lichens need clean air. They absorb whatever is dissolved in rainwater, so smoke and vehicle exhaust kill them. Plenty of lichens means clean air; almost none, on trees that otherwise look healthy, usually means polluted air. That is why scientists call lichens bio-indicators of air quality.
Try This: count the lichen patches on ten trees near a busy road and on ten trees inside a park. Compare the two counts — you have just measured air pollution without any instrument.
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