🎯 Learning outcomes
- Explain the water cycle with evaporation, condensation and precipitation
- Describe the composition of air and its uses
- Explain causes of water and air pollution and ways to conserve water
Before you start: Sources of water; air is around us.
📖 Explanation
Water moves in a cycle. The Sun heats water in seas, rivers and lakes; it evaporates into water vapour. High up, vapour cools and condenses into tiny droplets forming clouds. When droplets join and grow heavy, they fall as rain, hail or snow (precipitation), and flow back into rivers, seas and underground.
Air is a mixture of gases: about 78% nitrogen, 21% oxygen, small amounts of carbon dioxide, water vapour and dust. Oxygen is needed for breathing and burning; plants use carbon dioxide; moving air (wind) turns windmills and helps birds fly.
Sewage, factory waste and plastic pollute water; smoke from vehicles and burning pollutes air. We can conserve water by rainwater harvesting, drip irrigation, fixing leaks and reusing water.
Chennai made rooftop rainwater harvesting compulsory for buildings to refill groundwater after repeated water shortages.
✗ Water disappears when puddles dry up
✓ The water evaporates into the air as water vapour and later returns as rain.
🧪 Try this activity
Cover a glass of ice-cold water and watch droplets form on the outside. Explain where they came from (condensation of water vapour in the air).
✍️ Question bank with answers
Tap an option to check multiple-choice answers. Open “Answer” or “Marking points” for the others.
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Clouds form by
Answer
A. condensation
Water vapour cools into droplets.
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Which gas is most plentiful in air?
Answer
C. nitrogen
About 78% of air is nitrogen.
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Which saves water on farms?
Answer
D. drip irrigation
Drip irrigation gives water directly to roots.
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What gas do we need for breathing?
Answer
oxygen
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Name one cause of water pollution.
Answer
sewage, factory waste or plastic in rivers
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Why are wet clothes dried faster on a sunny, windy day?
Marking points
- Heat speeds evaporation
- Wind carries away the vapour
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Draw and explain the water cycle using the words evaporation, condensation and precipitation.
Marking points
- Sun heats water and it evaporates
- Vapour rises, cools and condenses into clouds
- Droplets fall as precipitation
- Water collects in rivers, seas and underground and the cycle repeats
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Suggest two ways your school can save water.
Answer
- Rainwater harvesting from the roof
- Fix leaking taps; use water from washing for plants
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Draw a pie chart of the gases in air.
Answer
- Nitrogen about 78%
- Oxygen about 21%
- Other gases about 1%
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