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NCERT Solutions Curiosity Chapter 8 and 154Water in a bottle cap and water in a plate (Tables 8.4 and 8.5) — Activity 8.7: Let us investigate

Book page 153 Updated on2026-09-05

Q1.
What conditions did we keep the same? What did we change in this investigation? What did we measure?
Answer

These three questions are the backbone of every fair test.

QuestionAnswer for Activity 8.7
What did we keep the same?The amount of water (same quantity in the cap and in the plate), the liquid used (water in both, or sanitiser in both), the place — both kept near each other, so the same temperature, the same air movement and the same humidity, and the starting time.
What did we change?Only one thing — the exposed area of the water. It is small in the bottle cap and large in the plate.
What did we measure?The time taken for the water to evaporate completely in each case.
Why the two must be kept side by side: if one were kept on a sunny window sill and the other in a cupboard, two things would have changed at once — area and temperature — and we could never say which of them caused the difference. Changing exactly one condition is what makes the test fair.
Tip: Measure the water with the same spoon or the same measuring cap for both. "About the same" is not good enough when you are timing the difference.
Q2.
Record the time taken for the water to completely evaporate in each case in Table 8.4. What can you conclude from this investigation?
Answer

Table 8.4, filled with typical readings for one teaspoon of water on a warm day (your own times will differ):

Exposed area of waterTime taken for complete evaporation
Less (bottle cap)About 3 hours
More (plate)About 35 minutes

Conclusion: water in the plate evaporates much faster than the same amount of water in the bottle cap. In the words of the book — if you spread out water on a plate, its area exposed to air is larger. Therefore, evaporation is faster.

Larger exposed area → more water particles at the surface → faster evaporation
Why area matters so much: particles can only escape from the surface of the water, not from deep inside it. In the cap, a small circle of water faces the air; in the plate, the same water is a wide thin film with many times that area. More surface means more escape routes, so the same amount of water leaves in a fraction of the time.
Where we use this every day: we spread wet clothes wide instead of leaving them folded; we spread grain, papads and chillies in a thin layer on the terrace; tea is poured into a saucer to cool it; and salt farmers spread sea water in wide shallow pans rather than deep tanks.
Q3.
What would happen if milk is taken instead of water in the above activity?
Answer

The same pattern would be seen — the milk in the plate would dry much sooner than the milk in the bottle cap — but there would be some interesting differences.

  • Milk is mostly water, so the water in it evaporates in exactly the same way, and the exposed area still decides the speed.
  • Milk would take a little longer than plain water, because the fat, protein and sugar in it form a skin on the surface that slows the escape of water particles.
  • The milk would not disappear completely. When all the water has gone, a dry whitish-yellow layer of solid milk residue is left behind — the fat, protein, milk sugar and minerals.
  • Left too long, that residue would smell sour, because milk spoils.
What this teaches: only the water in the milk evaporates. The solids dissolved and suspended in it stay behind. This is the same principle that gives us khoya (mawa) when milk is boiled down for hours, and salt when sea water dries in the salt pans.
Tip: If you actually try this, use a very small quantity and throw the residue away — do not taste it.
Q4.
Design an activity similar to Activity 8.7 to find out what are the other conditions which can affect how fast water will evaporate. What would you change? What would you keep the same? Use Table 8.5 to record the data.
Answer

Pick one condition, change only that, and keep everything else the same. Here are three good designs.

DesignCondition changedConditions kept the sameWhat you find
A. Air movementOne saucer under a running fan, one in still air in the same roomSame saucers, same amount of water, same room temperature, same humidity, started togetherThe saucer under the fan dries much sooner
B. TemperatureOne saucer on a sunny window sill, one in the shade of the same roomSame saucers, same water, same still airThe warmer saucer dries much sooner
C. HumidityOne saucer in the open room, an identical one covered by an upturned glass jarSame saucers, same water, same place, same temperatureThe covered saucer hardly dries — the air trapped in the jar soon becomes full of vapour

Table 8.5, filled in for Design A:

Condition that is kept the same: amount of water, size of the saucer, room temperature, humidity
Condition that is changedTime taken for complete evaporation
Still air (no fan)About 2 hours 30 minutes
Moving air (fan at full speed)About 45 minutes
Why moving air speeds evaporation up: the layer of air just above the water quickly fills with vapour and then blocks further escape. A fan or a breeze keeps sweeping that saturated layer away and brings fresh, drier air, so evaporation can continue at full speed.
Check it yourself: Design C is the most surprising one and needs nothing but a glass jar. It shows in the clearest possible way that humidity — the vapour already present in the air — is what finally stops evaporation.
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