Air We Breathe — Class 4 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 4 Science. What air is made of, what it can do, and how we are spoiling it.
1. Air is there even when you cannot see it
You cannot see air, so it is easy to think there is nothing around you. There is. Air is a mixture of gases wrapped around the whole Earth, and everything alive depends on it.
2. What air is made of
Air is not one gas but several, mixed together.
| Gas | How much | What it does |
|---|---|---|
| Nitrogen | about 78% | Dilutes the oxygen; plants need it to make proteins |
| Oxygen | about 21% | Supports breathing and burning |
| Argon | about 0.9% | An unreactive gas |
| Carbon dioxide | about 0.04% | Used by plants to make food |
| Other gases and water vapour | about 0.06% | Includes neon, helium and methane |
The surprise here is that oxygen is not the main gas. Oxygen is the one we cannot live without, but nitrogen is far more abundant. That is a good thing: nitrogen dilutes the oxygen. If the air were mostly oxygen, any small fire would blaze out of control.
3. What air can do
You can prove air is real with four simple observations.
Air occupies space. Push an empty upturned glass straight down into a bucket of water. The water does not fill it, because the trapped air is already taking up the room.
Air has weight. Air is not weightless. About a cubic metre of it weighs roughly 1.2 kg at sea level. Weigh a deflated balloon, then weigh it inflated, and the inflated one is very slightly heavier.
Air supports burning. Light a candle and cover it with a glass. The flame dies once the oxygen inside is used up.
Air is essential for life. Every animal and plant needs it. Humans can survive days without food and water, but only minutes without air.
4. Air pollution
Air becomes polluted when harmful gases and particles are added to it.
Four common causes:
- Vehicle exhaust from cars, buses, lorries and two-wheelers.
- Factory smoke from chimneys.
- Burning garbage and crop stubble in the open.
- Bursting firecrackers.
The main pollutants are PM2.5 and PM10 — tiny particles of dust and soot small enough to reach deep into the lungs — along with carbon monoxide, which reduces the blood's ability to carry oxygen, and sulphur dioxide and nitrogen dioxide, which cause acid rain.
The Air Quality Index (AQI) is a number that tells you how clean or dirty the air is on a given day. The higher the number, the worse the air.
5. How we can reduce it
What you can do:
- Walk or cycle for short journeys.
- Use public transport, or share a car ride.
- Never burn garbage or dry leaves.
- Say no to firecrackers.
- Plant trees and look after them.
What communities and governments can do: switch to electric vehicles, use solar and wind power, fit filters to factory chimneys, and keep proper waste collection running so that nothing needs burning.
6. Worked examples
Example 1. Which gas makes up most of the air?
Solution: Nitrogen, at about 78%.
Example 2. Why is it a good thing that air is not mostly oxygen?
Solution: Oxygen supports burning. If air were mostly oxygen, fires would spread uncontrollably. Nitrogen dilutes it.
Example 3. How would you show that air occupies space?
Solution: Push an upturned empty glass into water. The water cannot enter because air already fills the glass.
Example 4. A candle covered by a glass goes out. Why?
Solution: Burning needs oxygen. Once the oxygen trapped under the glass is used up, the flame dies.
Example 5. Name two causes of air pollution in a city.
Solution: Vehicle exhaust and factory smoke.
Example 6. What does a high AQI reading mean?
Solution: That the air quality is poor and unhealthy to breathe.
7. Practice
- Name the two most abundant gases in air and their percentages.
- Which gas in air supports burning?
- Which gas do plants use to make their food?
- Does air have weight? Give a rough figure.
- Name four causes of air pollution.
- What are PM2.5 particles, and why are they dangerous?
- What does AQI stand for?
- List three things you personally can do to reduce air pollution.
8. Answers
- Nitrogen about 78% and oxygen about 21%.
- Oxygen.
- Carbon dioxide.
- Yes. About 1.2 kg per cubic metre at sea level.
- Vehicle exhaust, factory smoke, burning garbage or stubble, and firecrackers.
- Very tiny dust and soot particles that can travel deep into the lungs and into the blood.
- Air Quality Index.
- Walk or cycle short distances, avoid firecrackers, and plant and care for trees.
9. Summary
- Air is a mixture of gases: about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide.
- Nitrogen is the most abundant gas, though oxygen is the one we need to breathe.
- Air occupies space, has weight, supports burning and is essential for life.
- Air pollution comes chiefly from vehicles, factories, open burning and firecrackers.
- PM2.5 and PM10 particles reach deep into the lungs; carbon monoxide cuts the blood's oxygen supply.
- The Air Quality Index reports how clean the air is; higher means worse.
- Walking, cycling, public transport, avoiding open burning and planting trees all help.
