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Nine pupils out of ten say the water put the flame out, or that iron rusts as soon as it gets wet. This lesson rewrites the causes: the flame dies because the oxygen is used up, and iron rusts only when oxygen and water are both present.
Objectives — By the end the pupil can: (1) state that air is about 78% nitrogen, 21% oxygen and 1% other gases, and explain why burning uses only about one fifth of it; (2) explain the water rise in the glass in three steps, oxygen used up, pressure falls, water pushed in, and say the rise is about one fifth of the glass; (3) design a three-nail control test (dry, tap water, salt water), say which condition each nail controls, and conclude that rust prevention means cutting off either oxygen or water.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Air exists | How do you show that air is real? | A bag holds air, wind moves paper, a balloon inflates |
| Watching a flame | What do you see while a candle burns? | Flame, heat, smoke, less wax |
| Fair test | How many things may you change in a fair test? | Only one; keep the rest the same |
02 Lesson flow (45 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-5 min | Diagnose | Ask why iron rusts and write answers on the board | Say the reason out loud |
| 5-12 min | Air pie chart | Draw a circle split 78 / 21 / 1 | Point to the oxygen slice |
| 12-22 min | Three acts | Teach composition, burning, then the two rust conditions | Repeat oxygen used up, flame goes out |
| 22-30 min | Candle reasoning | Run or show the glass experiment and ask why water rises | Give the three-step chain |
| 30-38 min | Nail design | Three nails: desiccant, tap water, salt water | Write a prediction with a reason |
| 38-45 min | Exit check | Collect three questions and mark at once | Name the error |
03 Teaching script (read it out)
Scene 1: Air composition: oxygen is only about one fifth
Ask what fills the room. Most pupils answer oxygen. Draw a circle and cut three slices: the biggest is nitrogen at about 78%, the next is oxygen at about 21%, and a thin strip is carbon dioxide, water vapour and other gases at about 1%.
Use a hundred-square strip instead: colour 78 squares nitrogen, 21 oxygen, 1 other. Make the pupil count the 21 and ask what fraction of a hundred that is: about one fifth.
Then draw the consequence: a sealed glass holds only enough oxygen to burn for a short while, and when those 21 parts are gone the flame has nothing to feed on.
Finish with one sentence from the pupil: most of the air is nitrogen, and only about one fifth is oxygen.
Use a hundred-square strip instead: colour 78 squares nitrogen, 21 oxygen, 1 other. Make the pupil count the 21 and ask what fraction of a hundred that is: about one fifth.
Then draw the consequence: a sealed glass holds only enough oxygen to burn for a short while, and when those 21 parts are gone the flame has nothing to feed on.
Finish with one sentence from the pupil: most of the air is nitrogen, and only about one fifth is oxygen.
Oxygen is only about one fifth of air, so a flame cannot last in a sealed space.
Scene 2: Burning needs oxygen: the three-step rise
Fix a candle to the base of a dish, add water, light the candle and lower a glass over it. The flame shrinks, goes out, and the water inside rises.
Tell it in three steps: first, burning used the oxygen inside the glass; second, with less oxygen the pressure inside drops; third, the outside air pressure pushes water in to take up the space.
The water rises about one fifth of the glass, which matches the 21 squares counted a moment ago. That is the same fact told twice, not a coincidence.
If the pupil stops at fire needs air, press the question: which part of the air, and why does water rise afterwards? Both layers, oxygen and pressure, must appear.
Tell it in three steps: first, burning used the oxygen inside the glass; second, with less oxygen the pressure inside drops; third, the outside air pressure pushes water in to take up the space.
The water rises about one fifth of the glass, which matches the 21 squares counted a moment ago. That is the same fact told twice, not a coincidence.
If the pupil stops at fire needs air, press the question: which part of the air, and why does water rise afterwards? Both layers, oxygen and pressure, must appear.
The flame dies because the oxygen is used up; the water rises because the pressure falls.
Scene 3: Rust needs oxygen and water together
Set up three nails. The first goes into a sealed jar with desiccant: air but no water. The second goes under boiled water covered with a layer of oil: water but no oxygen. The third sits in tap water: both air and water. Watch for a week.
Only the third rusts; the first two stay bright. Rusting needs oxygen and water at the same time, and either one missing stops it: desiccant removes the water, the oil film removes the oxygen.
Add a fourth nail in salt water. Salt does not cause rusting by itself, but it speeds the reaction, so the salt nail rusts fastest and thickest. Say this plainly, or pupils conclude salt is the cause.
From this, read off prevention: paint, grease, plastic coating and galvanising all put a barrier between the iron and the air or the water; stainless steel changes the metal itself so a protective layer forms.
Only the third rusts; the first two stay bright. Rusting needs oxygen and water at the same time, and either one missing stops it: desiccant removes the water, the oil film removes the oxygen.
Add a fourth nail in salt water. Salt does not cause rusting by itself, but it speeds the reaction, so the salt nail rusts fastest and thickest. Say this plainly, or pupils conclude salt is the cause.
From this, read off prevention: paint, grease, plastic coating and galvanising all put a barrier between the iron and the air or the water; stainless steel changes the metal itself so a protective layer forms.
Rust needs oxygen and water together; prevention means cutting one of them off.
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Composition | Which gas is most abundant in air? | Recall the biggest slice | Nitrogen, about 78% |
| Burning | Why does a candle under a glass go out? | The oxygen inside is used up | No oxygen left to burn |
| Experiment | Why does the water rise? | Oxygen used, pressure drops, air pressure pushes water in | About one fifth of the glass |
| Rust | Which nail rusts? | Check whether oxygen and water are both present | The tap-water nail; salt water rusts fastest |
Sanity check (teach this): Sense check: 78 plus 21 plus 1 must make 100, so any set that does not add to 100 is a recall error. A 500 mL glass of air holds about 105 mL of oxygen, so the water can rise only about one fifth; a full glass would be impossible because 395 mL of nitrogen and other gases are still there. Rust requires both water and air in the same place, so a bright nail kept over desiccant for a week is a correct result, not a failed experiment. The salt nail rusts faster but still needs water and oxygen, so salt causes speed, not rust.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| Composition | Draw the air pie chart with percentages | Nitrogen 78, oxygen 21, other 1; the three add to 100 |
| Experiment | The water rises one fifth of the glass. What does that show? | Oxygen is about one fifth of the air and was used up |
| Control | Design a test that rust needs water | One nail in tap water, one over desiccant, all else equal |
| Application | Why grease a bicycle chain? | The film keeps water and oxygen off the iron |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Treating air as pure oxygen | Air is all oxygen | Only about one fifth of air is oxygen. The rest is mostly nitrogen, which is why the flame dies so soon. |
| Wrong cause for the flame dying | The water put it out | No water reached the candle; the water rose outside it. The flame died because the oxygen ran out. |
| Rust blamed on water alone | Iron rusts when it gets wet | Wet is only half. The dry jar had air but no water and nothing happened. Both are needed. |
| Salt blamed as the cause | Salt makes iron rust | Salt only speeds it up. Salt water with no oxygen still will not rust. |
07 Homework (paper, section and question numbers)
- Primary 4 General Studies paper — Air and burning section: fill in the composition, then the candle reasoning question
- Primary 4 General Studies paper, materials — Short answer: give two ways to stop rusting and a reason for each
- Primary General Studies chapter index — All chapters in one table; revise the water cycle and dissolving
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Most abundant gas in air | Nitrogen, about 78% | Answering oxygen means the chart never landed; redraw it |
| How far does the water rise? | About one fifth of the glass | A full glass means the nitrogen was forgotten |
| State rust prevention in one line | Cut off oxygen or water so both cannot meet | Grease alone means the reason is missing; ask why |
09 Tutor FAQ
Must pupils memorise 0.04% carbon dioxide?
No. Primary 4 needs the three rough figures, 78, 21 and 1, and the word about. Precise values belong to secondary school.
Must the candle experiment be live?
Live is best when safety and ventilation allow; otherwise a video works, because the reasoning chain matters more than the apparatus.
The rust test takes a week. How does that fit one lesson?
Design and predict in the lesson, record the hypothesis, then compare photographs a week later. Designing the fair test is the assessed skill.
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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.