Primary General Studies: Air, Combustion And Rusting (Paper 3 (challenge))

Primary General Studies: Air, Combustion and Rusting (Paper 3 (challenge))

2026-11-13

Tutor Prep Guide · Food chains, producers and consumers, energy flow and ecological balance

Primary General Studies: Air, Combustion and Rusting (Paper 3 (challenge))

Paper three links the three elements of burning, rust prevention and cost calculations, with the last items needing several steps.

Direct answers　List what is given and what is asked before choosing a formula, keep the units on the answer, and use an estimate to check that it is reasonable.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: Primary General Studies: Air, Combustion and Rusting
Level	parts of a closed circuit, conductors against insulators, series against parallel, adding up battery voltage, and electrical safety

02　Questions (10 items)

FoundationWhich three things must be present for burning to happen?
FoundationPouring water on a camp fire mainly removes which element of burning?
FoundationWhy can rust not protect the iron underneath the way paint does?
IntermediateA sealed 1 000 mL bottle holds 210 mL of oxygen and a candle uses 15 mL a minute. How long can it burn?
IntermediateA 25 g piece of iron gains 40 per cent in mass when it rusts completely. What is its new mass?
IntermediateA 30 square metre fence needs 0.2 L of paint per square metre and paint costs $45 a litre. What is the total cost?
ChallengeWhy does galvanised iron resist rust better than painted iron?
Challenge100 g of iron becomes 140 g of rust. What percentage of the final mass is oxygen, to one decimal place?
ChallengeHow does burning iron wire in pure oxygen differ from burning it in air?
Error redoA classmate says putting out a fire only means taking away the fuel and the other two elements do not matter. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	A fuel, oxygen, and a temperature at or above the ignition point.	All three are needed, so putting out a fire means removing one of them.
2	It lowers the temperature below the ignition point.	Water takes in heat and turns to steam, so the fuel drops below its ignition point and the fire dies.
3	Rust is loose and porous, so water vapour and oxygen pass through and keep attacking the iron below.	Paint forms a tight film that keeps air and water out, while rust is crumbly and lets them through.
4	It can burn for 14 minutes.	210 divided by 15 is 14.
5	It weighs 35 g.	25 times 1.4 is 35, and the extra 10 g is oxygen.
6	The total is $270.	30 times 0.2 is 6 litres, and 6 times 45 is 270.
7	Zinc is more reactive than iron and oxidises first, so it still protects the iron even where the coating is scratched.	Zinc reacts with oxygen first and forms a protective layer, so the iron is not attacked.
8	About 28.6 per cent.	40 divided by 140 is 0.2857, which is about 28.6 per cent.
9	It burns much more fiercely and quickly, because the oxygen concentration is higher.	The higher the oxygen concentration, the faster the reaction, so iron wire throws out sparks in pure oxygen.
10	Removing any one of the three elements puts the fire out, and the best choice depends on the situation.	Smothering with sand or cooling with water works just as well, so the fuel does not always have to be removed.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

Paper four clears the conductor, insulator and battery direction misconceptions.Tutor Registration →

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