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NCERT Solutions Curiosity Chapter 10 .4 Do Plants Respire? — Activity 10.8: Let us find out (demonstration activity)

Book page 14910 Updated on2026-09-05

Q1.
Compare both the test tubes for any change in colour. Does the lime water turn milky in both the test tubes?
Answer

No. The lime water turns milky only in the test tube connected to the conical flask. In the other test tube, kept open beside it, the lime water stays clear.

Test tubeConnected to the flask of germinating seeds?Lime water after 24 hours
Test tube joined by the rubber pipeYesTurns milky
The other test tube (control)No — only ordinary air above itStays clear
Why a second test tube at all: it is the control. Ordinary air contains a very small amount of carbon dioxide, not enough to turn lime water milky in a day. The clear control tube proves that the milkiness in the first tube is caused by something coming from the flask.
Q2.
Why does the lime water turn milky in the test tube connected to the flask?
Answer

Because lime water turns milky due to the presence of more carbon dioxide in the flask.

When air from the flask is pushed through the glass tube into the lime water, the carbon dioxide it carries reacts with the lime water and forms a fine white insoluble solid, which makes the clear liquid look milky. The more carbon dioxide, the quicker and denser the milkiness.

Check it yourself: blow gently through a straw into a little lime water in a glass. It turns milky within seconds — the carbon dioxide of your own respiration doing exactly what the seeds' carbon dioxide does.
Q3.
But where does this carbon dioxide come from?
Answer

From the germinating moong seeds in the flask. Carbon dioxide is naturally present in very small quantities in the air; in the flask, additional carbon dioxide is produced by the seeds as they respire.

During respiration, glucose is broken down in the presence of oxygen, releasing carbon dioxide, water and energy:

Glucose + Oxygen → Carbon dioxide + Water + Energy

The energy produced is used by the plant for its growth and development — which is why soaked seeds sprout so vigorously.

Why the set-up is kept in the dark for 24 hours: in the dark the seeds cannot photosynthesise, so no carbon dioxide is used up. Everything collected in the flask is the product of respiration alone — that is what makes this a fair test of respiration.
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