S2 Science: Combustion, Oxidation And Mass (Paper 4 (error Clinic))

S2 Science: Combustion, Oxidation and Mass (Paper 4 (error clinic))

2026-11-14

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Tutor Prep Guide · The three conditions for burning, extinguishing methods, tests for carbon dioxide and oxygen, oxidation of magnesium and iron, complete and incomplete combustion, conservation of mass and calculating product masses

S2 Science: Combustion, Oxidation and Mass (Paper 4 (error clinic))

This paper corrects ten common errors about combustion and oxidation, covering whether burning needs oxygen, the type of change in rusting, the ash-is-lighter misconception, rusting conditions, mass gain when metals oxidise, the limewater test, mass in a sealed container, the source of carbon monoxide, the product of burning magnesium and the type of reaction.

Direct answers　Burning needs oxygen; rusting is a chemical change; lighter ash comes from escaping gases; rusting needs oxygen and water; metals get heavier when oxidised; limewater tests for carbon dioxide; mass in a sealed container is unchanged; carbon monoxide comes from incomplete combustion; magnesium gives magnesium oxide and gains mass; burning is oxidation with oxygen, so there are two reactants.

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: S2 Science: Combustion, Oxidation and Mass
Level	S2

02　Questions (10 items)

FoundationIs it true that burning always needs oxygen?
FoundationRusting is said to be only a physical change. What is wrong?
FoundationThe ash is lighter than the fuel, so mass is said to disappear. What is wrong?
IntermediateRusting is said to need only water. What is wrong?
IntermediateMetals are said always to become lighter after burning. What is wrong?
IntermediateMilky limewater is said to indicate oxygen. What is wrong?
ChallengeThe total mass is said to fall after burning in a sealed container. What is wrong?
ChallengeComplete combustion is said to produce carbon monoxide. What is wrong?
ChallengeThe white solid from burning magnesium is said to be ash lighter than the metal. What is wrong?
Error redoBurning is said to be a decomposition reaction needing only one substance. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Yes within this course, since oxygen is one of the necessary conditions for burning	With no oxygen there is no oxidation, so oxygen is part of the three conditions for burning.
2	Rusting forms a new substance, iron oxide, so it is a chemical change that cannot be wiped away	A physical change makes no new substance, while iron oxide has properties quite unlike iron.
3	Most products leave as gases, and the total mass has not disappeared	Add the escaping gases back and the mass equals what was there before.
4	Rusting needs both oxygen and water, and water alone does not cause it	That is why iron sealed in boiled distilled water hardly rusts.
5	Burning a metal forms an oxide, so added oxygen usually makes it heavier	It only looks lighter because the white oxide powder scatters easily.
6	Milky limewater indicates carbon dioxide, while oxygen is tested with a glowing splint that relights	The two gases have different tests and the tests are not interchangeable.
7	No gas can escape from a sealed container, so the total mass is the same before and after	Conservation of mass assumes no matter leaves the system.
8	Complete combustion gives carbon dioxide and water, while carbon monoxide comes from incomplete combustion when oxygen is short	Carbon monoxide is poisonous and forms most readily where ventilation is poor.
9	The white solid is magnesium oxide, which is heavier than the magnesium because oxygen has been added	4.8 g of magnesium gives 8.0 g of magnesium oxide, and the extra mass is oxygen from the air.
10	Burning is oxidation, in which a fuel combines with oxygen, so there are two reactants	Decomposition is the opposite, splitting one substance into two or more.

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.

For combustion and oxidation questions ask three things: whether the question is about conditions, observations or mass, how many reactants and products there are, and which gas entering or leaving explains any change in mass.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.