NCERT Solutions Curiosity Chapter 8 and 1488.2 Another Mystery — In-text Questions

Book page 147 Updated on2026-09-05

Q1.

Aavi and Thirav argued with a chain of reasons. What do you think about the possible reasons mentioned in Fig. 8.4?

Answer

Fig. 8.4 is a fine example of how scientific argument actually works — each statement is answered by a check, not by shouting.

Step in the chain	What it says	Is it a good move?
1	"Some of the water may have seeped out of the glass tumbler."	A fair starting guess — it can be tested.
2	"No, it cannot seep out. The level of water in the tumbler has not decreased."	Good — this brings evidence against the guess.
3	"It might have decreased, but may not be significant enough to be seen."	Honest and important — it points out a weakness in the measurement, not in the idea.
4	"With a tall and narrow bottle, even a slight change in the level of water is noticeable."	Excellent — it improves the method so that a small change would show up.
5	"We can take water at room temperature in another tumbler and find out whether any water seeps out."	Best of all — a control. If glass leaked, the warm tumbler would leak too.

What I think: every step is reasonable, and none of them is a personal attack. The children do not decide the answer by voting; they keep asking "how can we find out?" That is why the chain ends with a well-designed experiment. Step 5 is the key idea, because the room-temperature tumbler stays dry — proving that the glass does not leak and that the coldness is what matters.

Tip: Notice that a "wrong" idea was still useful. Ruling out seeping is what left condensation as the only surviving explanation.

Q2.

Where else have you seen water droplets like this? You might have seen dew drops on plants. Why do we see dew drops more in the morning?

Answer

Other places where the same droplets appear:

On a bottle of chilled water taken out of the refrigerator.
On the outside of a cold-drink bottle or a steel glass of lassi in summer.
On the inside of the windowpanes and the car windscreen on a winter morning.
On the mirror and tiles of the bathroom after a hot bath.
On spectacles when you come indoors from the cold, or when you breathe out with a mask on.
On the inner side of the lid of a cooking pot.
As mist on a cold steel spoon held above a cup of hot tea.

Why dew is seen more in the morning:

Through the night the leaves, grass and other objects lose heat to the open sky and become colder than the air around them.
The air near them holds water vapour.
When this moist air touches the cold leaf, the vapour condenses into tiny drops — dew.
The coldest hour is just before and around sunrise, so the dew is thickest then. As the Sun rises and warms everything, the drops evaporate and by mid-morning the grass is dry.

Why it is the same story as the tumbler: in both cases moist air meets a surface colder than itself. The tumbler is made cold by the ice; the leaf is made cold by losing heat all night. The vapour has nowhere to stay as a gas, so it turns liquid.

Did you know? Dew is heaviest on clear, still, cool nights. On a cloudy night the clouds send heat back to the ground, the leaves do not cool enough, and there is little dew in the morning.

Q3.

When we boil the water in a half-filled utensil and cover it with a steel plate, some water drops accumulate on the inner side of the steel plate. Where do these water drops come from? What do you think?

Answer

Those drops come from the water vapour rising from the boiling water. They are not splashes, and no water has come from outside.

Boiling water at 100 °C → water vapour rises
Vapour meets the cooler steel plate
Vapour loses heat → turns into liquid → drops on the inner side of the plate

Evidence that this is right:

The drops are on the inner (lower) side of the plate — the side facing the vapour — not on top.
They appear even if the plate was wiped bone dry before covering.
If you use a cold plate, drops form faster and in larger numbers; a plate already heated over the flame collects almost none.
Water in the pot slowly becomes less — that lost water is what you are collecting on the plate.

Why the plate has to be cooler: condensation needs the vapour to lose heat. A plate at room temperature is far below 100 °C, so vapour touching it cools instantly and turns back into water. If the plate were as hot as the steam, no drops would form.

Everyday use: This is exactly how an idli steamer, a bhapa dish and a rice cooker work — and it is the principle behind distillation, by which clean water is obtained from salty or muddy water.

Q4.

When the water vapour present in the air comes in contact with a cold surface, it forms water droplets. Could the water appearing on the outer surface of the glass tumbler in Activity 8.3 also be due to condensation of water vapour present in the air?

Answer

Yes. That is exactly what it is. The process is called condensation — the process of conversion of water vapour into its liquid state.

Why the tumbler is a perfect setting for it:

The ice keeps the water inside at about 0 °C, so the wall of the tumbler becomes very cold.
The air of the room always contains water vapour — that is its humidity.
Air touching the cold glass is chilled. Chilled air cannot hold as much vapour as warm air.
The extra vapour condenses on the glass as a film of tiny droplets, which merge into bigger drops and run down.

How the chapter proves it beyond doubt: two checks are made. (a) In Activity 8.4 the whole set-up is weighed on a digital balance and the reading increases — water has been added from the air, not lost from inside. (b) The water level inside is marked with a permanent marker and it does not go down, so nothing has leaked out. Together, these settle the matter.

Did you know? Condensation is the reverse of evaporation. Evaporation needs heat; condensation gives heat out. That is why the steam of a pressure cooker can scald you far more badly than boiling water at the same 100 °C.

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