NCERT Solutions Curiosity Chapter 8 and 149Weighing the condensation experiment on a digital balance — Activity 8.4: Let us measure

Book page 148 Updated on2026-09-05

Q1.

Predict what will happen to the mass of cold water kept on the digital weighing balance. Will it increase or decrease or remain the same?

Answer

Prediction: the reading will increase.

Think about what each of the three possible answers would mean:

If the reading…	…it would mean
decreases	Water is leaking out of the tumbler and dripping away, or water is escaping as vapour
stays the same	The water outside simply came from inside — nothing gained, nothing lost
increases	Water has been added from outside — from the air

Because the tumbler is covered with a steel plate, no vapour can escape from inside; and because condensation keeps depositing water from the air on the cold outer wall, the total mass on the pan goes up — slowly at first and then steadily, for as long as the tumbler stays cold.

Why making a prediction first matters: writing down what you expect before the reading changes turns the activity into a real test. If the mass had fallen, the seeping idea would have survived. It did not fall — so the prediction based on condensation was correct.

Q2.

Observe the reading on the balance and record the weight after every five minutes for 30 minutes in Table 8.2. Do your findings match with your predictions? Explain your observations.

Answer

Yes, the findings match the prediction — the mass increases. A typical set of readings looks like this (your own values will differ with the size of your tumbler and the humidity of the day):

Time	Mass of the set-up	Gain since the start
0 min	254.0 g	—
5 min	254.3 g	0.3 g
10 min	254.7 g	0.7 g
15 min	255.0 g	1.0 g
20 min	255.4 g	1.4 g
25 min	255.7 g	1.7 g
30 min	256.0 g	2.0 g

Explanation of the observations:

The tumbler and plate are cold. Water vapour from the air touches this cold surface and condenses into droplets on the outside.
That water was in the air before and is now on the balance pan — hence the reading rises.
The gain is roughly steady — about 0.3 to 0.4 g every five minutes — because condensation continues as long as the surface stays cold.
Once the ice has melted and the tumbler warms up towards room temperature, the gain slows down and finally stops.

Why the numbers are small: only a thin film of droplets forms, so the change is a fraction of a gram — which is exactly why a digital balance is used. An ordinary beam balance would show nothing at all, and you might wrongly conclude that nothing had happened.

Tip: Do not lift the tumbler between readings, and do not breathe over it. Both would spoil the measurement. Read the display without touching anything.

Q3.

There is an increase in the reading on the digital weighing balance. Can we conclude that water is not seeping through the wall of the tumbler? Can we also conclude that the water collected outside the tumbler is only due to condensation?

Answer

To the first question — not conclusively. To the second question — no. The book itself says, No, we cannot say that conclusively from Activity 8.4.

Why one experiment is not enough here:

The balance shows the mass of the whole set-up together — tumbler, water, ice, plate and the drops outside. If a little water seeped out through the glass, it would still be sitting on the pan and the total would not change because of it.
So the rise in mass proves that water has been gained from the air, but it cannot prove that no water is also seeping out at the same time. Both could be going on together.

What the experiment does prove: water has definitely been added from outside the system — condensation is certainly happening. That is a solid result. What it leaves open is whether seeping is happening as well. A good scientist reports exactly this much and no more.

Tip: This is a very useful habit — after every experiment ask, "what does my result rule out, and what does it leave open?" Claiming more than your data allows is one of the commonest mistakes in science.

Q4.

What more can you do to show that water is not seeping from the glass tumbler? How would you modify Activity 8.4 to find the answer? What do you observe, and what can you conclude from this?

Answer

The modification given in the book: repeat Activity 8.4 after marking the water level on the glass tumbler with a permanent marker or a strip of visible tape. Then watch the mark, not the balance.

What you observe:

The water level inside does not go down — it stays exactly at the mark (in fact it rises very slightly as the ice melts).
Meanwhile, more and more water collects on the outer surface of the tumbler.

Conclusion: water is not seeping out through the glass. The extra water on the outside is coming from the air, and it is getting collected there because of condensation.

Why the marker settles it: the previous experiment weighed everything together and so could not separate "gained from air" from "lost from inside". The mark looks only at the water inside. If seeping were happening, the level would have to fall. It does not fall — so seeping is ruled out.

Two further checks you can add: (1) use a tall, narrow bottle instead of a wide tumbler, so that even a tiny loss of water would show as a big drop in level; (2) keep an identical tumbler of room-temperature water beside it — it stays completely dry outside, proving that it is the cold surface, not the glass, that produces the drops.

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