🌬️ 1. What is Air?
Air is everywhere — inside buildings, in water, in soil, in our lungs. Although we cannot see it, air is a
mixture of gases that surrounds the Earth. This blanket of air around the Earth is called
the atmosphere.
Key facts about air:
- Air has NO colour, NO smell, NO taste
- Air occupies space (fill a balloon!)
- Air has weight/mass — it exerts pressure (atmospheric pressure)
- Air can be compressed (pressed into smaller space) — e.g., bicycle tyres, scuba tanks
- Air also exists in soil, water, and within living organisms
🧪 2. Composition of Air
Air is a mixture (not a compound) of several gases. The main components are:
| Component |
Approximate % |
Key role |
| Nitrogen (N₂) |
~78% |
Major component; used by plants (via bacteria in soil) to make proteins |
| Oxygen (O₂) |
~21% |
Essential for breathing (respiration) and combustion (burning) |
| Carbon Dioxide (CO₂) |
~0.04% |
Used by plants in photosynthesis; causes greenhouse effect |
| Water vapour (H₂O) |
Variable (0–4%) |
Part of the water cycle; causes weather, rain, humidity |
| Noble gases (Argon, etc.) |
~0.96% |
Inert; used in light bulbs, neon signs |
| Dust, pollutants, microbes |
Trace |
Part of air in practice (impurities) |
📸 Image Prompt
A clear pie chart showing the composition of air: The largest slice (about 78%)
labeled "Nitrogen (N₂) — 78%" in sky blue. The second largest (about 21%) labeled "Oxygen (O₂) — 21%" in
green. A tiny slice labeled "CO₂ — 0.04%" in grey. A tiny slice labeled "Noble gases + water vapour +
others — ~1%" in light orange. The pie chart title reads "Composition of Air". Each segment has a
colored legend box. The chart is bold, educational, and easy to read with percentage labels outside the
slice with leader lines. White background, clean infographic style.
Fig. 15.1 — Composition of air
🔥 3. Role of Air in Burning (Combustion)
Oxygen in air is necessary for burning. Without oxygen, nothing can burn.
Activity proof: Cover a burning candle with a glass jar → it goes out within a few seconds.
Why? The candle uses up all the oxygen inside the jar. Once oxygen is used up, combustion stops → flame
extinguishes.
Fire triangle: For anything to burn, you need:
(1) Fuel (wood, paper, petrol) + (2) Oxygen (from air) + (3) Heat (ignition) → Fire!
Remove any ONE of these → fire goes out!
🌿 4. Role of Air in Respiration and Photosynthesis
Respiration (All living things):
- Animals and plants take in Oxygen (O₂) and use it to break down glucose → release
energy
- They release Carbon Dioxide (CO₂) as waste
Photosynthesis (Green plants only):
- Plants absorb CO₂ from air through stomata
- Using sunlight energy, they combine CO₂ + water → glucose + Oxygen (O₂)
- The oxygen released by plants is what we breathe! This is why plants are called the "lungs of the
Earth"
Animals → breathe O₂, release CO₂. Plants → absorb CO₂ (day), release O₂. This is nature's perfect balance!
Cutting forests destroys this balance — less CO₂ absorption, more global warming, less O₂.
🐟 5. Air in Water and Soil
Air dissolved in water: Fish and other aquatic animals breathe using gills
that extract dissolved oxygen from water. This oxygen entered the water by dissolving from the air above.
When you boil water, you see bubbles — these are dissolved air escaping!
Air in soil: Soil contains air in tiny pores (gaps) between soil particles. Earthworms and
other soil organisms breathe this air. When it rains heavily, these pores fill with water → earthworms come
out to the surface to breathe!
🌍 6. The Atmosphere — Air's Protective Role
- The atmosphere protects Earth from harmful ultraviolet (UV) rays from the Sun (via the ozone layer)
- The atmosphere keeps Earth warm enough for life (greenhouse effect — in moderate amounts)
- Air resistance (drag) slows down falling objects and burns up meteors before they hit Earth
- Clouds (made of water vapour in air) cause rainfall → essential for life
Air Pollution
Causes: Vehicle exhaust (CO, CO₂, NOx), industrial smoke, burning of garbage, construction
dust, burning of fossil fuels, crop burning
Effects: Smog, respiratory diseases (asthma, lung cancer), global warming, acid rain, ozone
layer depletion
Solutions: Use public transport, plant trees, use CNG/electric vehicles, avoid burning
garbage, use cleaner fuels
📝 7. Quick Revision
- Air = mixture of gases. Cannot be seen, smelled, or tasted. Has mass, occupies
space
- Composition: ~78% N₂, ~21% O₂, ~0.04% CO₂, ~1% noble gases + water vapour
- O₂ is essential for burning (combustion) and respiration
- Candle goes out in enclosed jar → oxygen exhausted → proves O₂ needed for burning
- N₂ = most abundant; inert but used by plants via bacteria for making proteins
- CO₂ used by plants in photosynthesis; exhaled by animals in respiration
- Fish breathe dissolved oxygen in water using gills
- Earthworms come out of soil during heavy rains → waterlogged soil has no air in pores
- Atmosphere = blanket of air around Earth; protects from UV rays, keeps Earth warm
- Air pollution caused by vehicle exhaust, factories, burning garbage → causes
respiratory diseases, smog
