Water — Class 5 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 5 Science. Where the Earth's water is, how it moves, and how little of it we can actually drink.
1. How much water can we drink?
The Earth looks blue from space, so it seems absurd to worry about running out of water. The numbers explain why we do.
| Where the water is | Share |
|---|---|
| Oceans and seas (salty) | about 97% |
| Frozen in glaciers and ice caps | about 2% |
| Groundwater | about 0.6% |
| Rivers, lakes and the atmosphere | a tiny fraction |
Only about 3% of the Earth's water is fresh water, and most of that is locked up as ice. The water we can actually reach and drink is a very small slice indeed.
Sea water cannot be drunk. It is saline — so salty that drinking it makes the body lose more water than it gains.
2. Underground water
Rain that soaks into the ground is stored in porous layers of rock called aquifers. This is groundwater, and we reach it through wells and borewells.
As it seeps down through soil, sand and rock, it is naturally filtered. That is why underground water is free from suspended impurities such as mud and sand.
But filtered is not the same as safe. Groundwater can still carry dissolved salts, fluoride, arsenic or germs, none of which the soil removes. Water from a new borewell should be tested before anyone drinks it.
3. Desalination
Desalination is the artificial process of converting sea water into fresh water by removing the salt.
Given that 97% of the world's water is sea water, this sounds like the answer to everything. It is not, for one reason: desalination needs a great deal of energy and is therefore expensive. Chennai has desalination plants, but they supplement the city's supply rather than replacing rivers and rain.
4. The water cycle
The Earth's water is never used up. It circulates, endlessly.
Evaporation. The Sun heats water in oceans, rivers and lakes and turns it into water vapour. Plants add to it by releasing vapour from their leaves.
Condensation. The vapour rises, meets cooler air, and turns back into tiny droplets that gather as clouds.
Precipitation. When the droplets grow heavy enough, they fall as rain, snow or hail.
Collection. The water gathers in rivers, lakes and oceans, or seeps underground — and the cycle begins again.
5. Rainwater harvesting
Rainwater harvesting means collecting rain instead of letting it run away.
Rain falling on a rooftop is led through gutters and pipes, filtered through sand, gravel and charcoal, and then either stored in a tank or allowed to seep into the ground through a recharge pit.
The benefits are direct: it raises the groundwater level, refills wells that had dried, reduces flooding in the streets, and gives a supply that costs nothing to produce.
Tamil Nadu made rainwater harvesting compulsory for buildings, because its groundwater was being pumped out faster than the rain could replace it.
6. Saving water
- Close the tap while brushing your teeth or soaping your hands.
- Repair a leaking tap at once; a steady drip wastes many litres a day.
- Use a bucket rather than a running hose to wash a vehicle.
- Water plants with the water in which rice or vegetables were rinsed.
- Harvest rainwater at home and at school.
7. Worked examples
Example 1. What percentage of the Earth's water is in the oceans, and why can we not drink it?
Solution: About 97%. It is saline, so drinking it makes the body lose more water than it gains.
Example 2. Why is underground water free from suspended impurities?
Solution: Because it is naturally filtered as it seeps down through layers of soil, sand and rock.
Example 3. Is groundwater always safe to drink?
Solution: No. Filtering removes suspended matter but not dissolved salts, fluoride, arsenic or germs, so it should be tested.
Example 4. Define desalination and give its main drawback.
Solution: Converting sea water into fresh water by removing salt. It needs a great deal of energy and is expensive.
Example 5. Name the four stages of the water cycle.
Solution: Evaporation, condensation, precipitation and collection.
Example 6. Why is rainwater harvesting compulsory in Tamil Nadu?
Solution: Because groundwater was being pumped out faster than rain could replace it, so wells were drying up.
8. Practice
- How much of the Earth's water is fresh water, and where is most of it?
- What does saline mean?
- What is an aquifer?
- Which impurities does natural filtration through soil not remove?
- What is desalination?
- Name the four stages of the water cycle in order.
- Through what three materials is harvested rainwater usually filtered?
- Give three benefits of rainwater harvesting.
9. Answers
- About 3%, and most of that is frozen in glaciers and ice caps.
- Containing dissolved salt, as sea water does.
- A porous layer of rock underground in which water is stored.
- Dissolved substances such as salts, fluoride and arsenic, and germs.
- The artificial process of converting sea water into fresh water by removing salt.
- Evaporation, condensation, precipitation and collection.
- Sand, gravel and charcoal.
- It raises the groundwater level, refills dried wells and reduces street flooding.
10. Summary
- About 97% of the Earth's water is saline ocean water; only about 3% is fresh, and most of that is ice.
- Sea water cannot be drunk because it is saline.
- Groundwater is stored in aquifers and is naturally filtered of suspended impurities.
- Natural filtering does not remove dissolved substances or germs, so groundwater must be tested.
- Desalination converts sea water to fresh water but is energy-hungry and expensive.
- The water cycle runs evaporation, condensation, precipitation and collection.
- Rainwater harvesting collects roof runoff, filters it, and stores it or recharges the ground.
- It is compulsory in Tamil Nadu because groundwater was being drawn faster than rain replaced it.
