S2 Science: The Water Cycle And Sewage Treatment (Paper 4 (error Clinic))

S2 Science: The Water Cycle and Sewage Treatment (Paper 4 (error clinic))

2026-11-14

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Tutor Prep Guide · The processes of the water cycle, the ratio of fresh to salt water, the order of water and sewage treatment, and BOD with water saving figures

S2 Science: The Water Cycle and Sewage Treatment (Paper 4 (error clinic))

This paper reworks the commonest wrong answers, including filtering out salt, mixing up evaporation and condensation, the role of chlorine, hard water and treatment stages.

Direct answers　Each error should be named as a faulty concept, a reversed direction or a unit mistake.

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: The Water Cycle and Sewage Treatment
Level	S2

02　Questions (10 items)

FoundationA student answers that a water filter turns sea water into fresh water. What is wrong?
FoundationA student labels vapour cooling into droplets as evaporation. What is the correct process?
FoundationA student answers that chlorination removes every impurity. What is correct?
IntermediateA student says hard water is harmful and unfit to drink. What is correct?
IntermediateA student says primary sewage treatment already removes most organic matter. Which stage really does?
IntermediateA student says sewage treatment only protects the environment and has nothing to do with drinking water. What is wrong?
ChallengeA student says saving water stops the water cycle. What is correct?
ChallengeInfluent BOD is 400 mg/L and treatment removes eighty five per cent. What is the effluent BOD?
ChallengeHow much fresh water and how much salt come from distilling 250 kg of sea water containing 3.5 per cent salt?
Error redoA student thinks a higher BOD means cleaner water. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	A filter only removes insoluble solids while dissolved salt needs distillation or reverse osmosis	Particle size is the point: salt is dissolved as ions, which pass through filter pores.
2	Condensation, a change from gas to liquid	Evaporation goes liquid to gas, the opposite direction, so naming the start and end state avoids the mix up.
3	Chlorination only disinfects and removes neither grit nor dissolved salt	Each stage has its own job: sedimentation removes solids, filtration removes particles and disinfection kills germs.
4	Hard water is generally safe to drink but lathers poorly and forms scale	Hardness affects washing and plumbing rather than being a toxicity problem.
5	Secondary treatment, where microbes remove most organic matter	Primary only removes solids, so dissolved organic matter needs aeration and activated sludge.
6	Discharge pollutes rivers and reservoirs, so treating sewage protects the raw water we drink	Polluted source water raises treatment cost and risk, which links sewage works directly to drinking water.
7	Saving water only reduces human withdrawal while evaporation, condensation, rainfall and run-off continue	The cycle is driven by the Sun, so how much we use does not stop the natural flow.
8	60 mg/L	400 times 0.15 is 60, since removing eighty five per cent leaves fifteen per cent.
9	241.25 kg of fresh water and 8.75 kg of salt	250 times 0.035 is 8.75 kg of salt, and 250 minus 8.75 leaves 241.25 kg of fresh water.
10	A higher BOD means heavier organic pollution	BOD measures the oxygen used to break down organic matter, so higher values mean more pollution and less dissolved oxygen.

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 water cycle questions ask three things: which way the water changes state, whether the question asks about a quantity or a concentration, and whether the treatment is physical, biological or chemical.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.