P4 General Studies | Air, Burning and Rusting — Lesson Prep Guide (for tutors)

2026-11-11

Tutor lesson-prep guides · Primary General Studies

Pupils say air is oxygen because nobody ever asked them to measure it. This lesson uses one candle and 200 mL of air to turn 21% into a water level they can see.

Objectives — By the end pupils can: (1) state that air is about 78% nitrogen, 21% oxygen and 1% other gases, and say that burning uses only the oxygen; (2) explain why the water rises about one fifth (200 mL of air gives about 42 mL) and why the remaining 158 mL, mostly nitrogen, cannot support burning; (3) say that rusting needs both oxygen and water, and use four test tubes to explain which nail rusts and why the other three do not.

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	Why does water not enter an upside-down glass pushed into a basin?	Air is trapped inside and takes the space
Reading a measuring cylinder	A cylinder holds 200 mL, you pour out 40 mL, how much is left?	160 mL
Fair test	Two iron pieces, one wet one dry, what else must be the same?	All other conditions the same

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0–5 min	Diagnose	Ask what air is and write every answer on the board without judging	Say their idea and listen to others
5–12 min	Glass-in-water demo	Push an upside-down glass into a basin and ask why water stays out	Observe and say air is inside
12–22 min	Three scripts	Teach composition, burning using oxygen, and the two rusting conditions, pausing after each	Repeat: burning uses oxygen, water rises one fifth
22–32 min	Worked types	Model one question each on composition, water level, rusting conditions and prevention	Copy each step and give the reason
32–40 min	Independent practice	Hand out four questions, hint only: check for oxygen and water	Work alone and write reasons
40–45 min	Exit check	Collect three answers, mark together and note the error type	Say which step they got wrong

03 Teaching script (read it out)

Scene 1: Air is not empty: 78 to 21 to 1

Start with the upside-down glass: water cannot enter because air fills the space.
Write three numbers: nitrogen 78%, oxygen 21%, other gases 1%, and show that they add to 100%.
Oxygen is second, but it is the gas burning and breathing use. Nitrogen is the largest part yet does not support burning.
Have the pupil say it once: nitrogen is the largest part, oxygen is what burning uses.

Nitrogen is the largest part; burning uses oxygen.

Scene 2: Burning uses only the oxygen: 200 mL rises 42 mL

Fix a short candle in a basin, add water to half the candle, light it, then cover with a 200 mL bell jar or a large glass.
The candle goes out because the oxygen inside is used up. Water rises about 42 mL, roughly one fifth.
Work it out with the pupil: 200 times 0.21 is 42; the remaining 158 mL is mostly nitrogen, which does not support burning.
Ask for one sentence: the water rises one fifth because oxygen is one fifth of air.

Oxygen is one fifth, so the water rises one fifth.

Scene 3: Rusting needs oxygen and water: four tubes, seven days

Set four test tubes, each with one iron nail: (1) drying agent only, oxygen but no water; (2) water, open to air; (3) boiled water with a layer of oil, water but no oxygen; (4) salt water, oxygen and water with salt speeding it up.
Leave them for seven days and look once a day.
Only tubes 2 and 4 rust, and 4 rusts fastest.
Write the conclusion on the board: rusting needs oxygen and water, and removing either one stops it. Prevention means removing one: paint, oil, galvanising, stainless steel.

Rusting needs oxygen and water together; remove one and it stops.

04 Question types and worked steps

Type	Example	Steps	Answer
Air composition	Which gas is most abundant in air	Order them 78 to 21 to 1, the largest is nitrogen	Nitrogen, about 78%
Burning uses oxygen	A candle burns out in 200 mL of air, how far does the water rise	Oxygen is 21%, so 200 times 0.21	About 42 mL
Rusting conditions	Which of the four tubes rusts	It needs both oxygen and water; dry lacks water, oil seal lacks oxygen	The open water tube; salt water rusts fastest
Prevention	Why paint an iron gate	Paint keeps oxygen and water away from the iron	It blocks one of the two conditions

Sanity check (teach this): Check for reasonableness: oxygen is only about one fifth, so the water must rise far less than half; a pupil who says the jar fills completely thinks burning uses all the air. In 200 mL, 42 mL of oxygen plus 158 mL left must add back to 200. Rusting takes days, so any answer of one minute must be corrected, and salt water still needs hours.

05 Independent practice (3 questions)

Question	Answer	What to watch
Composition	What percentage of air is nitrogen and oxygen together	78 plus 21 is 99, the last 1% is other gases	99%
Burning	How much oxygen is in 500 mL of air	500 times 0.21 is 105	About 105 mL
Rusting	Why does the drying-agent tube not rust	Air is present but water is not, so one condition is missing	No water, so no rust
Prevention	Which method suits a bicycle chain	Oil spreads a layer that keeps water and oxygen off the surface	Oil

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating air as oxygen	Writing that air is oxygen and burning uses air	Air is only about one fifth oxygen; most of it is nitrogen, and nitrogen does not support burning.
Saying the water rises half way	Answering that water rises to half the jar	Oxygen is one fifth, so the water rises about one fifth; in 200 mL that is about 42 mL.
Thinking water alone causes rust	Saying wet means rust	The oil-sealed tube has water too, but no oxygen, so it stays clean for seven days. Both are needed.
Rust as germs	Saying paint kills the germs in iron	Rust is iron reacting with oxygen and water, not germs. Paint simply keeps air and water away.

07 Homework (paper, section and question numbers)

Primary General Studies chapter index (24 chapters) — Do the three questions in section 7 of this chapter first, then check the answers
Water cycle chapter (same subject) — Section 5 worked types: three states and the four-step cycle, same idea of looking at conditions
Dissolving and separation chapter (same subject) — Section 7 independent practice: separation methods, practising the idea of keeping things apart

08 Exit check (last 5 minutes)

Question	Answer	Decision
Which gas is most abundant in air	Nitrogen, about 78%	Answering oxygen means the ratio is not secure, retake script one
A candle burns out in 200 mL of air, how far does the water rise	About 42 mL, one fifth	Answering 200 mL means oxygen is not seen as one fifth, redo the demo
Which two conditions does an iron nail need to rust	Oxygen and water	Water alone means the four-tube comparison must be repeated

09 Tutor FAQ

Must P4 pupils memorise 78, 21 and 1?

They should know that nitrogen is the largest part and oxygen is about one fifth, because schools ask which gas is most abundant and which supports burning. Writing 78% and 21% is enough; decimals are not needed.

What if there is no bell jar?

A large drinking glass over a short candle in a basin works the same way, and the water still rises about one fifth. What matters is that pupils see the size of the rise, not the equipment.

Are rusting and burning related?

Both react with oxygen; burning is fast and rusting is slow. Pupils only need to say both need oxygen. Do not teach equations at this level.

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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.