S2 Science: Combustion, Oxidation And Mass (Paper 3 (challenge))

S2 Science: Combustion, Oxidation and Mass (Paper 3 (challenge))

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 3 (challenge))

This paper centres on conservation of mass and practices word equations, exothermic reactions and putting out oil fires, then uses fixed mass ratios to calculate carbon dioxide, sulphur dioxide, water and the composition of rust.

Direct answers　Carbon plus oxygen gives carbon dioxide; burning is exothermic; sand smothers an oil fire; 12 g of carbon with 32 g of oxygen gives 44 g of carbon dioxide; magnesium to oxygen is 24 to 16; atoms are only rearranged so mass is unchanged; 8 g of sulphur gives 16 g of sulphur dioxide; 4 g of hydrogen gives 36 g of water; 20 g of rust with 6 g of oxygen holds 14 g of iron and is 30 percent oxygen; 4 g of oxygen can burn only 0.5 g of hydrogen.

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)

FoundationWrite the word equation for the complete combustion of carbon.
FoundationIs burning exothermic or endothermic?
FoundationWhy does sand put out a burning oil fire?
IntermediateWhat mass of carbon dioxide forms when 12 g of carbon reacts completely with 32 g of oxygen?
IntermediateWhat is the mass ratio of magnesium to oxygen in magnesium oxide?
IntermediateWhy does the number of atoms not change in a chemical reaction?
ChallengeHow much sulphur dioxide forms when 8 g of sulphur burns completely?
ChallengeHow much water forms when 4 g of hydrogen burns completely?
ChallengeA 20 g sample of rust contains 6 g of oxygen. What mass of iron is present and what percentage is oxygen?
Error redoIt is said that 4 g of hydrogen and 4 g of oxygen react completely. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Carbon + oxygen -> carbon dioxide	Reactants go on the left of the arrow and products on the right, with ignition as the condition.
2	Exothermic, since burning gives out heat and light	That is why burning can cook food and generate electricity, turning chemical energy into heat and light.
3	The sand cuts off oxygen from the air, so the flame dies for lack of oxygen	Oil floats on water and would splash, so sand is used rather than water.
4	44 g	By conservation of mass, 12 plus 32 equals 44, so the total mass of reactants equals that of products.
5	24 to 16, that is 3 to 2	This fixed ratio lets you work out the mass of magnesium oxide from the mass of magnesium.
6	Atoms are only rearranged, never destroyed or created	Because the atoms are unchanged in number, the total mass is unchanged, which is conservation of mass.
7	16 g	Sulphur and sulphur dioxide are in the ratio 32 to 64, so 8 g of sulphur takes 8 g of oxygen to give 16 g.
8	36 g	Hydrogen, oxygen and water are in the mass ratio 4 to 32 to 36, so 4 g of hydrogen needs 32 g of oxygen.
9	14 g of iron, and oxygen is 30 percent	Subtract 6 from 20 for the iron, and 6 divided by 20 times 100 gives the oxygen percentage.
10	Hydrogen and oxygen react in the ratio 4 to 32, so 4 g of oxygen can burn only 0.5 g of hydrogen and hydrogen is left over	The mass ratio of reactants is fixed, so equal masses do not both get used up.

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.