Water — Class 4 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 4 Science. The water cycle, making water safe to drink, pollution and conservation.
1. Water never runs out, but it does move
The water on Earth today is the same water that has always been here. It is not made or destroyed; it simply travels round and round in the water cycle.
2. The four stages of the water cycle
1. Evaporation. The Sun heats water in oceans, rivers, lakes and ponds, and it turns into an invisible gas called water vapour which rises into the sky.
Evaporation does not happen only from the sea. It happens from every wet surface — puddles, wet clothes, even your skin when you sweat. Plants add to it too, releasing water vapour from their leaves.
2. Condensation. High up, the air is colder. The rising vapour cools and turns back into tiny droplets, and millions of these droplets together form clouds.
3. Precipitation. When the droplets grow too heavy to float, they fall as rain, or as snow or hail in colder places.
4. Collection. The fallen water gathers in rivers, lakes and oceans, or seeps into the ground as groundwater — and the cycle begins again.
3. Making water safe to drink
Water from a river or a lake is not fit to drink as it is. A waterworks cleans it in three stages.
| Stage | What happens |
|---|---|
| Sedimentation | Water stands in large tanks so that heavy particles such as sand and mud settle to the bottom |
| Filtration | Water passes through layers of sand, gravel and charcoal, which trap smaller particles and some germs |
| Chlorination | A small measured amount of chlorine is added to kill the remaining germs |
At home, boiling water for a few minutes kills germs, and a filter removes particles.
4. Water pollution
Water becomes polluted when harmful substances are added to it. There are three main sources:
- Domestic sewage — untreated waste water from houses let into rivers.
- Industrial waste — factories releasing chemicals, heavy metals and hot water.
- Agricultural runoff — fertilisers, pesticides and animal waste washed off farms by rain.
Polluted water spreads diseases such as cholera, typhoid and diarrhoea, and it kills the fish and plants that live in it.
5. Groundwater is not unlimited
It is tempting to think of groundwater as a separate, endless store beneath our feet. It is not. Groundwater is rainwater that has soaked into the ground. If we pump it out faster than the rain replaces it, the water table falls and wells run dry — which is exactly what has happened across much of Tamil Nadu.
Rainwater harvesting is the answer: collecting rain from rooftops and letting it seep back into the ground. It is compulsory for buildings in Tamil Nadu for this reason.
6. Saving water
- Close the tap while brushing your teeth or soaping your hands.
- Repair leaking taps at once — a dripping tap can waste many litres a day.
- Use a bucket rather than a running hose to wash a vehicle.
- Reuse the water in which rice or vegetables were washed to water plants.
- Harvest rainwater at home and at school.
7. Worked examples
Example 1. Name the four stages of the water cycle in order.
Solution: Evaporation, condensation, precipitation and collection.
Example 2. What forms a cloud?
Solution: Millions of tiny water droplets formed when rising water vapour cools and condenses.
Example 3. In which stage of purification do heavy particles settle?
Solution: Sedimentation.
Example 4. Why is chlorine added to drinking water?
Solution: To kill the germs that remain after filtration.
Example 5. Name two sources of water pollution.
Solution: Domestic sewage and industrial waste.
Example 6. Why do wells run dry even where it still rains?
Solution: Because groundwater is pumped out faster than rainwater can seep back to replace it.
8. Practice
- What is evaporation?
- Name the stage in which water vapour turns into clouds.
- Name the three stages of water purification.
- What is done in the filtration stage?
- Name three sources of water pollution.
- Name two diseases spread by polluted water.
- Where does groundwater come from?
- Give three ways to save water at home.
9. Answers
- Water turning into water vapour when heated by the Sun.
- Condensation.
- Sedimentation, filtration and chlorination.
- Water passes through sand, gravel and charcoal, which trap smaller particles and some germs.
- Domestic sewage, industrial waste and agricultural runoff.
- Cholera and typhoid (also diarrhoea).
- From rain that seeps down into the ground.
- Close the tap while brushing, repair leaks promptly, and use a bucket instead of a hose.
10. Summary
- Water is never lost, only moved, through the water cycle.
- The four stages are evaporation, condensation, precipitation and collection.
- Evaporation happens from every wet surface, not only from the sea.
- Water is purified by sedimentation, then filtration, then chlorination.
- Pollution comes from domestic sewage, industrial waste and agricultural runoff.
- Polluted water spreads cholera, typhoid and diarrhoea.
- Groundwater is stored rainwater and can be used up faster than it is replaced.
- Rainwater harvesting and everyday care both matter.
