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Pupils have all heard of air, but ask what is inside it and the answer is only oxygen. This lesson turns air into something measurable, then narrows burning and rusting to the same key: oxygen.
Objectives — Pupils can: (1) state the main gases in air and their rough shares (78% nitrogen, 21% oxygen, about 1% other); (2) use the bell-jar experiment, where water rises about one fifth, to explain that burning uses up oxygen; (3) name the three conditions for burning and use one of them to explain a way of putting a fire out; (4) explain that rust needs both oxygen and water, and give four ways of preventing it.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Air takes up space | Push an empty cup upside down into water. Does water get in? | No, because the air inside holds the space |
| Observing burning | What do you see when a candle goes out? | The flame disappears, with smoke and a smell |
| Fractions and percentages | What is one fifth of 100 mL? | 20 mL |
02 Lesson flow (45 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-6 min | Two opening questions | Ask what is inside air and how we could prove air has weight; sort pupils answers on the board, keeping the wrong ones visible | Give their own answer and suggest a way to test it |
| 6-16 min | What air is made of | Write 78% nitrogen, 21% oxygen, about 1% other; for a 500 mL sample that is 105 mL oxygen and 390 mL nitrogen | Work out the oxygen and nitrogen in a 500 mL sample themselves |
| 16-26 min | Bell-jar demonstration | Burn a candle under a bell jar standing in water; water rises about one fifth; ask pupils to explain why | Explain that the oxygen is used up and water fills the gap |
| 26-32 min | Three conditions and firefighting | Write fuel, oxygen and enough heat; match each on to a way of putting a fire out (lid, sand, extinguisher) | Name the three conditions and apply one to a real case |
| 32-40 min | Rust needs two things | Set up three test tubes: drying agent, boiled water with oil, water with salt; explain rust as iron reacting with oxygen and water | Explain why tube two does not rust and what the salt does |
| 40-45 min | Prevention and exit check | Four ways to prevent rust: paint, oil, galvanising, stainless steel; then the three exit questions | Name two methods and the condition each one blocks |
03 Teaching script (read it out)
Scene 1: Air is a mixture, not simply oxygen
The air we breathe is a mixture: about 78% nitrogen, about 21% oxygen, and about 1% argon, carbon dioxide and water vapour, plus traces of other gases. In a 500 mL sample that is 500 times 21%, roughly 105 mL of oxygen, and 500 times 78%, roughly 390 mL of nitrogen; together 495 mL, leaving about 5 mL of everything else. That number matters. If air were pure oxygen a single match would flare like a firework; because so much nitrogen dilutes the oxygen, burning stays gentle and we can use fire safely in a kitchen. Pupils who think of air as oxygen only cannot explain the bell-jar result, because they expect all the air to be used up.
Have pupils restate: air is a mixture, 78% nitrogen and 21% oxygen, so 500 mL holds about 105 mL oxygen.
Scene 2: Burning takes only the oxygen and leaves a space
Stand a burning candle in a shallow dish of water and lower a bell jar over it. The flame dims and dies and the water inside rises, stopping at roughly one fifth of the jar. Burning has used up the oxygen, oxygen is about one fifth of air, and the space it leaves is filled by water. The same demonstration proves two things at once: burning needs oxygen, and oxygen is about one fifth of air. The important detail is where the water stops. It rises about one fifth and no further, because the remaining nitrogen neither burns nor dissolves away. If water fills the whole jar, the jar is leaking or the gas inside was pure oxygen, and the experiment, not the theory, is what is wrong.
Have pupils restate: water rises about one fifth because oxygen is about one fifth of air, then the flame stops.
Scene 3: Rusting is slow oxidation and also needs oxygen and water
Rust is what happens when iron reacts with oxygen and water to form reddish-brown hydrated iron oxide. The layer is flaky and porous, so it falls off and exposes fresh iron underneath. Because both oxygen and water are needed, the three-tube comparison is designed as controls. Tube one holds a drying agent and is sealed, so no water means no rust. Tube two holds water that has been boiled to drive out dissolved oxygen, with a layer of oil on top to keep air out, so no oxygen means no rust. Tube three holds ordinary water and shows rust within days; add salt and rusting speeds up sharply because salt water conducts electricity and helps the reaction along. Preventing rust means cutting one condition: paint and oil shut out water and air, while galvanising and stainless steel work by letting a less damaging metal corrode first, protecting the iron underneath.
Have pupils restate: rust needs oxygen and water, salt speeds it up, and prevention blocks one of the two.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Calculate a gas share | A 1,000 mL sample of air holds roughly how much oxygen? | (1) oxygen is about 21%; (2) 1,000 times 0.21; (3) check that the answer is smaller than the nitrogen share | About 210 mL of oxygen, and about 780 mL of nitrogen |
| Explain the water rise | In the bell jar the water rose 20 mL. How much oxygen was used, and what does that show? | (1) the volume that rose equals the oxygen used; (2) compare with the whole jar; (3) state the share | About 20 mL of oxygen, so in a 100 mL jar oxygen is about one fifth |
| Judge a rusting setup | A nail sits sealed in a jar with a drying agent. Is it rusty after a month? Why? | (1) check for water; (2) check for oxygen; (3) both are needed | No rust, because with no water the conditions are not met even though oxygen is present |
| Choose a prevention method | A bicycle chain rubs against itself and gets wet. Which method fits? | (1) list the conditions (rubbing, water); (2) rule out paint, which wears off; (3) choose oil | Oil, because it lubricates and blocks water and air; paint would rub away |
Sanity check (teach this): Check the arithmetic: the oxygen volume is always about one fifth of the total and the nitrogen share is over three times the oxygen share, so a nitrogen figure smaller than the oxygen figure means the two were swapped. In the bell-jar explanation the water can never rise above the jar volume, and a jar that fills completely has leaked. When naming a prevention method, always say which condition it blocks; paint on its own, with no reason, is an incomplete answer.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| A nail stands in boiled water with a layer of oil on top. Rust after seven days? | No: boiling drove out dissolved oxygen and the oil keeps air out, so oxygen is missing | Whether they named both the boiling and the oil layer, and what each one blocks |
| Repeat the bell-jar experiment with pure oxygen. Where does the water stop? | Almost at the top of the jar, because nearly all the gas can be used up | Whether they reasoned from the oxygen share instead of recalling the usual answer |
| Why does drying a kitchen knife matter? Answer with the rusting conditions. | Drying removes water, so one of the two conditions is missing and the reaction slows sharply | Whether the everyday habit was linked back to the two conditions |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Thinking air is only oxygen | Air is just oxygen, that is what we breathe. | Air is a mixture: 78% nitrogen, 21% oxygen, about 1% other gases. Burning uses the oxygen and the nitrogen stays behind. |
| Expecting the jar to fill completely | Why did it not fill up? Did the experiment fail? | Oxygen is only one fifth of air, so the water stops at about one fifth. Filling completely means a leak or pure oxygen. |
| Thinking water alone causes rust | It got wet, that is why it rusted; drying it stops the problem. | Rust needs oxygen and water together. A nail in boiled water under oil stays bright precisely because oxygen is missing. |
| Memorising firefighting tricks | Putting a fire out means a lid, water or sand; just remember the three. | Match each to a condition: a lid or sand cuts off oxygen, water removes heat, and water on a chip-pan fire spreads it. |
07 Homework (paper, section and question numbers)
- Primary 4 General Studies Paper, Part A Matter and Energy, Question 3 — Part A, Question 3: air composition and burning; pupils add one sentence explaining the water rise
- Primary 4 General Studies Paper, Part B Everyday Applications, Question 7 — Part B, Question 7: rusting and prevention; pupils name the condition each method blocks
- Primary General Studies chapter index — Follow the index to the water cycle and Hong Kong weather chapters to connect water across topics
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| What share of air is oxygen? | About 21%, roughly one fifth | If they say a half, go back to the 78 and 21 figures |
| What are the three conditions for burning? | Fuel, oxygen and enough heat | If they only say a flame is needed, redo the three conditions |
| What does rust need? | Both oxygen and water; salt speeds it up | If they say damp, add the oxygen condition explicitly |
09 Tutor FAQ
Which year is this for?
Written for Primary 4. Primary 3 can stop at observing and describing the bell jar, leaving percentage calculations for later.
Is the candle experiment safe?
Use a short candle, a shallow tray and a clear plastic jar, and let the tutor light and place everything; pupils observe and record only.
Is there a matching paper?
Parts A and B of the Primary 4 General Studies paper both carry relevant questions; the homework section names the parts and question numbers.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.