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NCERT Solutions Curiosity Chapter 8 and 156Two bottle caps of water, one in sunlight and one in shade (Fig. 8.6) — Activity 8.8: Let us explore

Book page 155 Updated on2026-09-05

Q1.
Place one cap in sunlight and keep the other in shade, observe them after every 15 minutes and record the time taken for the water to completely evaporate in each case. What conclusions can you draw from Activity 8.8 and other similar experiences?
Answer

Typical readings for equal amounts of water in two identical caps on a clear summer day:

Where the cap is keptTime for complete evaporation
In sunlightAbout 1 hour 15 minutes
In shadeAbout 4 hours

Conclusions, in the words of the book:

  • Water evaporates faster from the cap kept in sunlight compared to the cap kept in shade.
  • It is a common observation that clothes dry faster on a hot sunny day.
  • With the increase in the movement of air, water evaporates faster — clothes dry faster on a windy day.
  • On a rainy day water evaporates slowly, because the air is already high in humidity.
Why sunlight makes such a large difference: sunlight warms both the water and the cap. Warmer water means faster-moving particles, and a far greater number of them can break away from the surface each second. Notice that the two caps were identical and held the same amount of water, and were watched at the same 15-minute intervals — so temperature was the only condition that differed.
Tip: Do not simply write "the sun dries it". Say which condition changed — the temperature of the water — and that everything else was kept the same. That is what turns an observation into a conclusion.
Q2.
It is a common observation that clothes dry faster on a hot sunny day. Do clothes dry faster or slower on a windy day?
Answer

Faster. Clothes dry more quickly on a windy day — often faster than on a still, hot day.

More movement of air → the moist layer above the cloth is swept away → faster evaporation

Why the wind helps so much:

  1. As water evaporates, a thin layer of very humid air forms right against the wet cloth.
  2. If that layer stays there, it soon becomes full of vapour and evaporation almost stops.
  3. Wind blows this saturated layer away and replaces it with fresh, drier air.
  4. So the surface of the cloth is always facing dry air, and water keeps leaving it at full speed.
Note the important difference from sunlight: the wind does not add heat — a breeze is usually cooler than still air. It speeds up drying by removing the vapour, not by warming the water. This is a very common misunderstanding, and question 5 of the exercises is built on it.
Everyday proof: this is why washing is hung in an open, breezy place rather than in a closed bathroom, and why the same clothes on the same day dry faster on the terrace than on a balcony sheltered by walls.
Q3.
If you want to dry your clothes on a rainy day, how can you make it faster?
Answer

On a rainy day the air is already very humid, so we must work on the other three conditions — area, air movement and temperature.

What to doWhich condition it uses
Wring or spin-dry the clothes hard firstRemoves most of the water before drying even begins
Spread each garment out fully on a wide line or a stand, never bunched or foldedIncreases the exposed area
Hang them apart with gaps between garmentsLets air reach both sides
Switch on a fan or hang them in a through-draught between two open windowsIncreases air movement and sweeps away the humid layer
Dry them in a warm room, or press them with a hot iron, or use a hair dryer or a machine dryerRaises the temperature
Do not dry them in a small closed bathroomThe trapped air becomes saturated and drying stops
Why humidity is the villain here: if the amount of water in the air is already high (more humidity), water evaporates slowly. The air simply has little room left to take in more vapour. We cannot change the weather, but a fan and a well-spread garment can still get the job done.
Do you know? The amount of water vapour in the air is more on rainy days, and hence rainy days are more humid. That is also why a rainy day feels sticky — sweat does not evaporate, so our body cannot cool itself easily.
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