# Chapter 7 Revision: Living Things and Air

F.2 Integrated Science · Chapter 7 · Lesson 4 · Revision of 7.1–7.6

The whole chapter before the test: gas tests, respiration and photosynthesis, the starch test, the balance of gases, air pollution and smoking.

## Key points

- **Air**: helium fills balloons and airships because it is **not flammable**. Nitrogen is unreactive, so it has no simple test.
- **Respiration ／ 呼吸作用** uses oxygen to break down food to release energy, forming carbon dioxide. It takes place in every living cell, at all times.
- **Breathing** is the exchange of air in and out of the lungs: nasal cavity → trachea → bronchus → branches of bronchus → air sacs, where **gas exchange** occurs.
- **Photosynthesis ／ 光合作用**: green plants make their food under sunlight from water and carbon dioxide. **Chlorophyll ／ 葉綠素** absorbs the light; leaves store the food as **starch**.
- **Starch test**: boil the leaf in hot water (kills the leaf cells) → alcohol (removes the chlorophyll) → hot water (softens the leaf) → iodine solution. Before the experiment, **destarch** the plant in darkness.
- **A leaf needs light and chlorophyll**: a part covered by aluminium foil, or a part that is not green, makes no starch.
- **Producers** (green plants) make food by photosynthesis; **consumers** (all animals) live on other living things.
- **Balance**: burning and respiration add carbon dioxide to the atmosphere; photosynthesis takes it in. More carbon dioxide is the main cause of the enhanced **greenhouse effect ／ 溫室效應**.
- **Air pollutants**: sulphur dioxide (can cause asthma and bronchitis), nitrogen dioxide, and carbon monoxide (lowers the amount of oxygen carried by blood).
- **Smoking**: cigarette smoke contains tar, nicotine (causes addiction) and carbon monoxide. **Passive smoking** is breathing in smoke from other smokers.

## Must know

| Test for | Use | Result |
|-----|------|--------|
| Oxygen | Glowing splint | Relights |
| Carbon dioxide | Limewater | Colourless → milky |
| Carbon dioxide | Hydrogencarbonate indicator (red) | More carbon dioxide: yellow · less: purple · no change: red |
| Water | Dry cobalt chloride paper | Blue → pink |
| Starch | Iodine solution | Brown → dark blue (blue-black) |
| Nitrogen | No simple test | The gas is unreactive |

A tube with indicator only is the **control**: it shows that the results are due to the factor under investigation only.

## Common mistakes

- ✗ "Breathing occurs in every cell of our body." ✓ **Respiration** occurs in every living cell. Breathing is the exchange of air in and out of the lungs.
- ✗ "Destarching is done by boiling the leaf in alcohol." ✓ Destarching is keeping the plant in darkness to remove the starch. Alcohol removes the **chlorophyll**.
- ✗ "Carbon dioxide turns yellow hydrogencarbonate indicator red." ✓ More carbon dioxide turns the red indicator **yellow**; less carbon dioxide turns it **purple**.
- ✗ "Green plants keep carbon dioxide and oxygen in balance by respiration." ✓ By **photosynthesis**, which takes in carbon dioxide and gives out oxygen.
- ✗ "Oxygen is used as fuel during respiration." ✓ Food is broken down; oxygen is **needed** for respiration to release the energy in food.

## Quick check

1. Which test shows that a gas is oxygen, and which shows water vapour? State each result.
2. In the starch test, why is the leaf put into alcohol, and why into hot water afterwards?
3. Two leaves are sealed in tubes of red hydrogencarbonate indicator. Tube X is left in bright light; tube Y is wrapped in aluminium foil. What colour does each indicator become?
4. Name two processes that add carbon dioxide to the atmosphere and one that takes it in.
5. The walls of a smoker's air sacs have broken down. Why does the smoker feel short of breath?

## Answers

1. Oxygen: a glowing splint relights. Water vapour: dry cobalt chloride paper turns from blue to pink.
2. Alcohol removes the chlorophyll; hot water washes away the alcohol and makes the leaf soft again.
3. X: purple (carbon dioxide decreases). Y: yellow (carbon dioxide increases).
4. Burning and respiration add it; photosynthesis takes it in.
5. There are fewer, larger air sacs and a smaller surface area for gas exchange, so less oxygen diffuses into the blood.

## Did you know?

Your trachea is held open by about 16–20 C-shaped rings of cartilage; you can feel them at the front of your neck.