Primary General Studies: Air, Combustion And Rusting (Paper 4 (error Clinic))

Primary General Studies: Air, Combustion and Rusting (Paper 4 (error clinic))

2026-11-13

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

Primary General Studies: Air, Combustion and Rusting (Paper 4 (error clinic))

Paper four drills the six kinds of item that lose the most marks: gas proportions, rusting conditions, changes in mass, rate comparisons and variable control.

Direct answers　Write the known conditions first, then the working, then use an estimate to check that the size of the answer makes sense.

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)

FoundationAbout what proportion of air is oxygen?
FoundationWhich two substances must be present together for iron to rust?
FoundationWhich two main substances are produced when a candle burns?
IntermediateAbout how many litres of oxygen are in 300 litres of air?
IntermediateA 40 g piece of iron weighs 48 g after rusting. By what percentage did the mass increase?
IntermediateWhy do silica gel packs inside a biscuit tin reduce rusting of the tin?
ChallengeWhy should only one variable be changed at a time in a controlled rusting experiment?
ChallengeThe same nail rusts in 5 days in salt water and 15 days in fresh water. How many times faster is salt water?
ChallengeA sealed 600 mL bottle holds 126 mL of oxygen and a candle uses 9 mL a minute. How long can it burn?
Error redoA classmate says iron gets lighter when it rusts. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	About 21 per cent.	Oxygen is about 21 per cent of air and nitrogen about 78 per cent.
2	Oxygen and water.	Neither can be missing, and that is the basis of every rust prevention method.
3	Carbon dioxide and water.	The wax reacts with oxygen to give carbon dioxide and water vapour.
4	About 63 litres.	300 times 0.21 is 63.
5	It increased by 20 per cent.	8 divided by 40 is 0.2, which is 20 per cent.
6	They absorb moisture, and iron rusts far less without water.	Rusting needs water, so taking the moisture away removes one of the conditions.
7	With just one difference you can tell which factor caused the result.	Change two things at once and you cannot tell which one made the difference.
8	It is 3 times faster.	15 divided by 5 is 3.
9	It can burn for 14 minutes.	126 divided by 9 is 14.
10	Oxygen joins the iron during rusting, so the mass actually increases.	Rust is iron combined with oxygen, so adding oxygen makes the total mass larger, which is why rusting bridges swell.

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.

With all four done, go back to the chapter page and revise the three parts of a closed circuit plus three safety habits.Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.