Air — Class 3 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 3 Science. What air is, what it does, and how we keep it clean.
1. Air is real, even though you cannot see it
Air is colourless, so you cannot see it. It is odourless, so pure air has no smell. It is tasteless as well.
Because of this, many people think air is nothing at all. It is not. Air is made of gases, and it behaves like other things you can hold.
Air occupies space. Push an empty glass upside down into a bucket of water. The water does not fill the glass, because air is already inside and will not move out of the way. Tilt the glass slightly and bubbles escape; only then does water go in.
Air has weight. Weigh a deflated balloon, then blow it up and weigh it again. The inflated one is heavier, because it now holds more air.
Air presses. That press is what keeps a balloon round and a cycle tyre firm.
2. What air is made of
Air is not one gas but a mixture.
| Gas | How much |
|---|---|
| Nitrogen | about 78 parts in 100 |
| Oxygen | about 21 parts in 100 |
| Argon | about 1 part in 100 |
| Carbon dioxide and others | a very small amount |
So most of the air around you is nitrogen, not oxygen. Oxygen is the part living things need, but it is only about a fifth of the whole.
3. What air does for us
- Breathing. Every living thing needs oxygen. Fish take theirs from oxygen dissolved in water.
- Burning. A fire needs oxygen. Cover a burning candle with a glass and it goes out once the oxygen inside is used up.
- Moving things. Windmills turn, kites fly and sailing boats move, all pushed by air.
- Carrying seeds. Many seeds and grains of pollen travel on the wind to new ground.
We do not breathe out only carbon dioxide. The air we breathe out still holds a good deal of oxygen, which is exactly why mouth-to-mouth rescue breathing works at all.
4. Wind is moving air
Moving air has different names depending on how strongly it blows.
| Name | Strength |
|---|---|
| Breeze | gentle, pleasant |
| Wind | stronger, moves branches |
| Storm or gale | very strong, can damage roofs and trees |
Calling every moving air "wind" hides that difference, and the difference matters when a warning is given.
5. Keeping air clean
Air becomes polluted when harmful things are mixed into it.
Causes: smoke from vehicles and factories, burning garbage and crop stubble, dust from building work, and bursting crackers.
Effects: coughing, asthma and other breathing illnesses, and damage to plants and buildings.
What helps: planting trees, cycling or taking a bus instead of a private vehicle, and saying no to crackers.
6. Worked examples
Example 1. An upside-down glass is pushed into water and the water does not rise inside. Why?
The glass is not empty. It is full of air, and air occupies space, so the water cannot enter until the air is let out.
Example 2. Which gas is most plentiful in air, and which one do we need for breathing?
Nitrogen is most plentiful at about 78 parts in 100. Oxygen, at about 21 parts, is the one needed for breathing.
Example 3. A candle is covered with a glass jar and goes out after a few seconds. Explain.
Burning needs oxygen. The jar traps only a small amount of air, and once that oxygen is used up the flame cannot continue.
Example 4. Give two ways a school could reduce air pollution.
Plant trees in the grounds, and encourage children to walk, cycle or share a bus rather than come by private vehicle.
7. Practice
- Name three properties of air that you cannot detect with your eyes, nose or tongue.
- Describe one experiment that shows air occupies space.
- What fraction of air, roughly, is oxygen?
- Why does a fire go out when covered?
- Give the three names for moving air, from gentlest to strongest.
- List three causes of air pollution.
- Name two things a family could do to reduce the air pollution they cause.
8. Answers
- It is colourless, odourless and tasteless.
- Push an inverted glass into water; the water does not enter because air already fills the glass.
- About one fifth, or 21 parts in 100.
- Because burning needs oxygen, and covering the flame cuts off the fresh supply.
- Breeze, wind, storm or gale.
- Vehicle smoke, factory smoke, burning garbage or crop stubble; construction dust and crackers also count.
- Use a bicycle or bus instead of a private vehicle, and avoid bursting crackers.
9. Summary
- Air is colourless, odourless and tasteless, but it is matter: it occupies space, has weight and presses.
- An inverted glass in water keeps the water out because air is already inside it.
- Air is a mixture: about 78 parts nitrogen, 21 parts oxygen, and small amounts of others.
- Air is needed for breathing and burning, and moving air drives windmills and carries seeds.
- Breathed-out air still contains oxygen, not only carbon dioxide.
- Moving air is a breeze, a wind or a storm, depending on strength.
- Pollution comes from smoke, burning and dust; trees and shared transport reduce it.
