# 7.1 Air and its Properties · 7.2 Respiration

F.2 Integrated Science · Chapter 7 · Lesson 1

What is in the air, how to test for each gas, and how living things release energy from food.

## Key points

- Air is a **mixture**: nitrogen **78 %**, oxygen **21 %**, noble gases **0.93 %**, carbon dioxide about **0.04 %**, plus a variable amount of water vapour.
- **Nitrogen**: food packaging, liquid nitrogen stores living cells, fertilizers. **Oxygen**: cutting and welding metals, breathing aid, supports burning. **Carbon dioxide**: fire extinguishers, dry ice (sublimes at −78.5 ℃).
- **Noble gases ／ 惰性氣體** are colourless, odourless and unreactive: helium (balloons, diving gas), neon (neon lights), argon (light bulbs, preserving old documents).
- **Breathed air** vs **unbreathed air**: oxygen 21 % → 16 %, carbon dioxide 0.04 % → 4 %, nitrogen stays 78 %, water vapour becomes saturated, and it is warmer.
- **Respiration ／ 呼吸作用** releases the energy in food inside living cells: food + oxygen → carbon dioxide + water + energy.
- Like burning, respiration uses oxygen and gives out carbon dioxide, water and energy. Unlike burning, it is slow, at body temperature, and gives no light.
- With glucose as the food: C~6~H~12~O~6~ + 6O~2~ → 6CO~2~ + 6H~2~O.

## Must know

| Gas | Test | Result if the gas is present |
|---|---|---|
| Oxygen | Glowing splint | Relights |
| Carbon dioxide | Limewater | Colourless → milky |
| Carbon dioxide | Hydrogencarbonate indicator | Red → yellow |
| Water vapour | Dry cobalt chloride paper | Blue → pink |
| Nitrogen | No simple test (it is unreactive) | – |

Hydrogencarbonate indicator: **purple** (less CO~2~ than air) · **red** (same as air) · **yellow** (more CO~2~).

## Common mistakes

- ✗ "Breathing is respiration." ✓ **Breathing** moves air in and out of the lungs. **Respiration** is the chemical process inside cells that releases energy from food.
- ✗ "Respiration gives out heat and light, like burning." ✓ Heat only. It happens slowly, step by step, at body temperature.
- ✗ "A glowing splint tests for carbon dioxide." ✓ A glowing splint **relights in oxygen**. Carbon dioxide turns **limewater milky**.
- ✗ "Breathed air contains no oxygen." ✓ It still contains about 16 % oxygen. That is why mouth-to-mouth resuscitation works.
- ✗ "Cobalt chloride paper turns pink to blue when wet." ✓ Dry paper is **blue**; water turns it **pink**.

## Quick check

1. Which gas makes up about four-fifths of the air? Give one use of it.
2. A gas turns limewater milky. Which gas is it, and which second test could confirm it?
3. Write the word equation for respiration.

## Answers

1. Nitrogen (78 %): filling food packages, storing living cells as liquid nitrogen, or making fertilizers.
2. Carbon dioxide. Hydrogencarbonate indicator turns from red to yellow.
3. food + oxygen → carbon dioxide + water + energy (in living cells).

## Did you know?

Helium was discovered on the Sun before it was found on Earth. In 1868 astronomers noticed an unknown colour line in sunlight and named the gas after helios, the Greek word for Sun. It was not found on Earth until 1895.