Water — Class 3 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 3 Science. Sources of water, safe drinking water and saving it.

1. Where water comes from

Rain is the original source of all fresh water on Earth. Every well, river, lake and pond is filled, directly or indirectly, by rain.

2. Most of the Earth's water cannot be drunk

The Earth looks well supplied with water, and most of it is sea water full of dissolved salt.

Only about three parts in every hundred of the Earth's water is fresh, and much of even that is frozen or deep underground. What is actually available for drinking, cooking and farming is a small share of a small share.

That is the reason saving water matters even in a place where it rains.

3. Potable water

Potable water is water that is safe to drink: clean, free of germs and free of harmful chemicals.

Ways to make water potable:

• Boiling is the simplest, and it kills the germs.
• Filtering removes dirt and solid particles.
• Chlorination adds a small measured amount of chlorine to kill germs.

Boiling does not make sea water drinkable. Boiling kills germs, but the salt stays behind and in fact becomes more concentrated as water leaves as steam. Removing salt takes a different process altogether.

4. Saving water at home

• Turn the tap off when the bucket is full instead of letting it overflow.
• Wash fruits and vegetables in a bowl, not under a running tap.
• Turn the tap off while brushing your teeth.
• Wash a vehicle with a bucket rather than a running hose. A hose can use ten times as much water for the same job.
• Repair a dripping tap. A slow drip wastes a surprising amount over a week.

5. Looking after ponds, lakes and rivers

• Deepen ponds and lakes, so they hold more water through the dry season.
• Plant trees on the banks. Roots hold the soil, so less of it washes in and fills the water body up.
• Keep rubbish out. Plastic thrown into a river lasts for hundreds of years, harms fish, and blocks the channels that carry water away in heavy rain.

6. Worked examples

Example 1. Why can sea water not be made drinkable by boiling?

Boiling kills germs but does not remove dissolved salt. As steam leaves, the remaining water becomes saltier still.

Example 2. Which is the largest source of water on Earth, and can it be drunk?

The sea. It cannot be drunk, because it is salt water.

Example 3. A family washes their scooter with a running hose. Suggest a better method and say why.

Use a bucket. A hose left running can use many times the water a bucket needs for the same wash.

Example 4. Name two ways to make water from a well safe to drink.

Boil it, which kills the germs, or filter it and then chlorinate it.

7. Practice

1. What is the original source of all fresh water?
2. Roughly what fraction of the Earth's water is fresh?
3. What does potable mean?
4. Name three ways of making water potable.
5. Why does boiling not help with sea water?
6. Give three ways to save water at home.
7. Give two ways to look after a village pond.

8. Answers

1. Rain.
2. About three parts in every hundred.
3. Safe to drink: clean, free of germs and harmful chemicals.
4. Boiling, filtering and chlorination.
5. Because it removes germs but not the dissolved salt.
6. Do not let buckets overflow, wash vegetables in a bowl, and turn the tap off while brushing.
7. Deepen it so it holds more water, and plant trees on the banks so soil does not wash in.

9. Summary

• Rain is the source of all fresh water; wells, rivers, lakes and ponds all come from it.
• Only about three parts in a hundred of the Earth's water is fresh.
• The sea is the largest store but is salt water and not drinkable.
• Potable water is safe to drink; boiling, filtering and chlorination all help.
• Boiling kills germs but cannot remove salt.
• Save water by using buckets, closing taps and mending drips.
• Deepening ponds, planting trees on banks and keeping plastic out protect water bodies.