Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
What is Air?
Air is a mixture of gases that surrounds the Earth. We cannot see it, but we can feel it as wind.
Composition of Air
| Gas | Approximate Percentage | Main Role |
|---|---|---|
| Nitrogen (N₂) | 78% | Most abundant; supports plant growth (through soil bacteria) |
| Oxygen (O₂) | 21% | Respiration, combustion (burning) |
| Carbon dioxide (CO₂) | 0.04% | Photosynthesis in plants |
| Other gases | ~1% | Argon, neon, helium |
| Water vapour | Variable | Part of water cycle; affects weather |
| Dust particles | Small amount | Causes pollution; important for cloud formation |
Nitrogen is the most abundant gas in air (not oxygen — a common mistake).
Properties of Air
- Air has weight — a balloon filled with air is heavier than a deflated one.
- Air occupies space — pushing an empty bottle into water shows bubbles (air escapes).
- Air exerts pressure in all directions.
- Air is colourless, odourless, and tasteless (pure air).
Uses of Oxygen
- Respiration — all living organisms breathe oxygen to get energy.
- Combustion — fire needs oxygen to burn. (Cover a flame with a lid → no oxygen → fire goes out.)
- Rusting — iron rusts in the presence of oxygen and water.
Uses of Nitrogen
- Makes up 78% of air but is relatively inactive (inert).
- Used in fertilisers — soil bacteria fix atmospheric nitrogen into nitrates for plants.
- Used to preserve food (inert packaging).
- Liquid nitrogen used for freezing (medical and food industry).
Uses of Carbon Dioxide
- Photosynthesis — plants use CO₂ to make food.
- Fire extinguishers use CO₂ — it does not support burning.
- Used in carbonated drinks (fizzy drinks, soda).
- Too much CO₂ → greenhouse effect → global warming.
Wind
- Wind is moving air.
- Caused by unequal heating of different parts of Earth by the Sun.
- Hot air rises (less dense) → cool air moves in to take its place → wind is formed.
- Uses of wind: windmills (generate electricity, pump water), sailing, flying kites, pollination of some flowers.
How Aquatic Animals Breathe
- Fish, aquatic insects, and underwater animals cannot breathe air directly.
- Water contains dissolved oxygen (O₂ dissolved in water).
- Fish use gills to extract dissolved oxygen from water.
- That is why aquariums need air pumps — to dissolve more oxygen in water so fish can survive.
Oxygen in Soil
- Soil also contains air in tiny pores between soil particles.
- This air provides oxygen to organisms living in soil (earthworms, bacteria, insect larvae).
- Waterlogged soil has less air — earthworms come to the surface during heavy rain to breathe.
Key Vocabulary
| Term | Meaning |
|---|---|
| Atmosphere | Layer of air surrounding the Earth |
| Nitrogen | Most abundant gas in air (78%) |
| Oxygen | Gas needed for breathing and burning (21%) |
| Carbon dioxide | Gas used by plants for photosynthesis |
| Combustion | Burning (needs oxygen) |
| Wind | Moving air |
| Dissolved oxygen | Oxygen dissolved in water (used by aquatic animals) |
| Gills | Organ used by fish to extract dissolved oxygen from water |
Exam Tips
- Nitrogen is 78%, Oxygen is 21% — numbers are often asked.
- Most abundant gas = Nitrogen (NOT oxygen — this is the most common mistake).
- Aquatic animals breathe dissolved oxygen using gills — not air directly.
- CO₂ has two roles: plant photosynthesis AND greenhouse effect — mention both when asked about CO₂.
- Wind = moving air; forms because hot air rises and cool air rushes in.
Frequently Asked Questions
Air is a mixture of gases. Main components: Nitrogen (~78%), Oxygen (~21%), Carbon dioxide (~0.04%), and other gases like argon, water vapour, and dust particles. Nitrogen is the most abundant gas in air.