Air Around Us — Class 6 Physical Science
'Air is invisible but essential — every breath connects us to the atmosphere.'
1. Introduction
Air is everywhere around us. We cannot see it, smell it (usually), or taste it, but it is real. Air is a mixture of gases that forms the Earth's atmosphere.
Why This Chapter Matters
- Understand what we breathe
- Learn about the atmosphere
- Air pollution awareness
- Foundation for environmental science and chemistry
2. Is Air Really There?
Even though air is invisible, we can prove its presence:
Experiments to Show Air Exists
| Experiment | Observation | Proof |
|---|---|---|
| Blow into a balloon | Balloon inflates | Air fills the balloon |
| Wave a paper fan | You feel air on your face | Moving air is felt |
| Open a bottle underwater | Bubbles come out | Air escapes from bottle |
| A falling leaf moves sideways | Leaf does not fall straight | Moving air (wind) pushes it |
| Place an empty glass upside down in water | Water does NOT enter fully | Air inside the glass blocks water |
'Air is real — it takes up space, has weight, and can be felt when it moves.'
3. Composition of Air
Air is NOT a single gas — it is a mixture of several gases.
Major Components
| Gas | Percentage | Symbol | Key Property |
|---|---|---|---|
| Nitrogen | ~78% | N₂ | Most abundant; relatively unreactive |
| Oxygen | ~21% | O₂ | Supports burning and respiration |
| Carbon dioxide | ~0.04% | CO₂ | Plants use for photosynthesis |
| Argon | ~0.93% | Ar | Noble gas (unreactive) |
| Other gases | ~0.03% | — | Neon, helium, methane, etc. |
| Water vapour | Varies (0-4%) | H₂O | Amount depends on humidity |
Pie Chart Representation
Nitrogen: ████████████████████████████ (78%)
Oxygen: █████████ (21%)
Others: ██ (1%)
4. Properties of Air
| Property | Explanation | Demonstration |
|---|---|---|
| Has weight | Air has mass | A balloon weighs more when inflated |
| Takes up space | Air occupies volume | Blowing up a balloon, bubbles in water |
| Exerts pressure | Air pushes on surfaces | Suction cup sticks, straw works |
| Expands on heating | Air becomes less dense | Hot air balloon rises |
| Is compressible | Can be squeezed | Pumping a bicycle tyre |
| Transparent | Light passes through | We can see through air |
5. Uses of Air Components
Oxygen (O₂)
The most important gas for life.
| Use | Explanation |
|---|---|
| Respiration | All living things need oxygen to release energy from food |
| Burning | Fire needs oxygen — without it, fire goes out |
| Medical | Oxygen cylinders for patients with breathing problems |
| Welding | Oxygen + fuel creates a hot flame for cutting/welding metals |
Nitrogen (N₂)
The most abundant gas — surprisingly unreactive!
| Use | Explanation |
|---|---|
| Fertilisers | Ammonia from nitrogen is used to make fertilisers |
| Packaging | Nitrogen gas is used to pack chips (keeps them fresh — no oxygen for spoilage) |
| Bulbs | Light bulbs are filled with nitrogen (prevents filament from burning) |
| Diluent | Dilutes oxygen — pure oxygen would be too reactive |
Carbon Dioxide (CO₂)
| Use | Explanation |
|---|---|
| Photosynthesis | Plants use CO₂ and water to make food |
| Fire extinguishers | CO₂ is heavier than air — smothers fires |
| Fizzy drinks | CO₂ is dissolved in soda, cola, beer |
| Dry ice | Solid CO₂ used for cooling |
Water Vapour (H₂O)
| Use | Explanation |
|---|---|
| Weather | Humidity affects rainfall and climate |
| Respiration | Moisture in breath; plants release water vapour |
6. Oxygen and Burning
The Fire Triangle
Fire needs THREE things:
- Fuel (wood, paper, LPG, etc.)
- Oxygen (from air)
- Heat (to reach ignition temperature)
fire
/ │ \
Fuel O₂ Heat
'Remove any ONE of the three and the fire goes out.'
How to Put Out a Fire
| Method | What It Removes | Example |
|---|---|---|
| Cover with a blanket | Removes oxygen | Small kitchen fire |
| Use water | Removes heat | Wood fire |
| Use a CO₂ extinguisher | Removes oxygen | Electrical fire |
| Sand on an oil fire | Removes oxygen | Kitchen oil fire |
Experiment: Candle and Glass
- Light a candle
- Cover it with a glass jar
- After some time, the candle goes out
- Reason: The oxygen inside the glass is used up, and fresh oxygen cannot enter
7. Respiration and Air
Air in Respiration
- We breathe in air (inhale): 21% oxygen, 0.04% CO₂
- We breathe out air (exhale): 16% oxygen, 4% CO₂
- Our body uses oxygen and produces CO₂
Comparison: Inhaled vs Exhaled Air
| Component | Inhaled Air | Exhaled Air |
|---|---|---|
| Oxygen | ~21% | ~16% |
| Carbon dioxide | ~0.04% | ~4% |
| Nitrogen | ~78% | ~78% |
| Water vapour | Variable | Saturated (more) |
| Temperature | Normal | Warm |
Test for CO₂
- Blow through a straw into lime water (calcium hydroxide solution)
- Lime water turns milky/cloudy → CO₂ is present
- This is a standard test for carbon dioxide
8. Air Pollution
Air can be polluted by harmful substances.
Sources of Air Pollution
| Source | Pollutants |
|---|---|
| Vehicle exhaust | CO, NO₂, smoke |
| Factory smoke | SO₂, smoke, dust |
| Burning of garbage | Smoke, toxic gases |
| Construction sites | Dust |
| Agricultural burning | Smoke, particulate matter |
Effects of Air Pollution
- Health problems: Asthma, bronchitis, lung cancer, heart disease
- Acid rain: SO₂ and NO₂ react with water → acidic rain (harms plants, buildings)
- Global warming: CO₂ and methane trap heat
- Damage to buildings: Acid rain damages marble (Taj Mahal is affected)
How to Reduce Air Pollution
- Use public transport (buses, trains)
- Carpooling
- Plant more trees (absorb CO₂)
- Use clean energy (solar, wind)
- Avoid burning garbage
- Regular vehicle maintenance
AP Context
- Visakhapatnam: Industrial city with some air quality concerns
- Vijayawada: Rapid urbanisation → increased vehicular pollution
- Good news: AP has a high forest cover in some districts (Eastern Ghats) that helps clean the air
9. Common Mistakes — Fix Them Now
| # | Mistake | Correction |
|---|---|---|
| 1 | Thinking air is mostly oxygen | Air is 78% NITROGEN, only 21% oxygen |
| 2 | Believing we breathe in only oxygen | We breathe in ALL components of air (N₂, O₂, CO₂, etc.) |
| 3 | Calling air 'nothing' because it's invisible | Air is real — it has weight, occupies space, exerts pressure |
| 4 | Thinking plants only take in CO₂ | Plants also need oxygen for respiration (they do BOTH: photosynthesis in daylight, respiration always) |
| 5 | Covering an oil fire with water | WATER SPREADS OIL FIRE! Use sand or a lid to smother it. |
10. Exam Focus
Marks Blueprint
| Question Type | Marks | Topic |
|---|---|---|
| MCQ | 1 | Air composition / property |
| Short answer | 2 | Prove air exists / oxygen and burning |
| Short answer | 2 | Components and their uses |
| Long answer | 3 | Air pollution / respiration |
| Activity | 2 | Candle and glass experiment |
Quick Self-Test (5 Questions)
Q1: What is the most abundant gas in air?
<details><summary>Answer</summary>Nitrogen (~78%).</details>Q2: Why does a candle go out when covered with a glass?
<details><summary>Answer</summary>The oxygen inside gets used up for burning, and fresh oxygen cannot enter.</details>Q3: How can you test for carbon dioxide?
<details><summary>Answer</summary>Blow exhaled air through lime water — it turns milky/cloudy if CO₂ is present.</details>Q4: What three things are needed for fire?
<details><summary>Answer</summary>Fuel, oxygen (air), and heat (ignition temperature).</details>Q5: How does inhaled air differ from exhaled air?
<details><summary>Answer</summary>Inhaled: 21% O₂, 0.04% CO₂. Exhaled: 16% O₂, 4% CO₂. Exhaled air has less O₂, more CO₂, and more water vapour.</details>11. Chapter Summary
- Air composition: 78% N₂, 21% O₂, 0.04% CO₂, plus water vapour and other gases
- Properties: has weight, takes up space, exerts pressure, transparent, compressible
- O₂: needed for respiration and burning
- N₂: used in fertilisers, packaging, bulbs
- CO₂: used by plants for photosynthesis, in fire extinguishers, fizzy drinks
- Fire triangle: Fuel + Oxygen + Heat → Fire
- Inhaled vs exhaled: less O₂, more CO₂, more water vapour in exhaled air
- Air pollution: causes health problems, acid rain, global warming
'Air is our most essential resource — we can survive weeks without food, days without water, but only minutes without air.'
