S2 Science: The Water Cycle And Sewage Treatment (Paper 3 (challenge))

S2 Science: The Water Cycle and Sewage Treatment (Paper 3 (challenge))

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 3 (challenge))

This paper covers softening hard water, infiltration against run-off, the environmental point of tertiary treatment, and calculations on leaks and evaporation.

Direct answers　Hardness is either temporary or permanent, paving increases run-off, tertiary treatment removes nutrients, and area times depth gives volume.

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)

FoundationWhich two ions are present in larger amounts in hard water?
FoundationName two ways of softening hard water.
FoundationWhat is the release of water vapour from plants into the air called?
IntermediateAfter rain reaches the ground, how do infiltration and run-off differ?
IntermediateA tap drips once a second at about 0.05 mL a drop. How much is lost in a day and in a 30 day month?
IntermediateFlushing takes three tenths of a 150 L household use. How much is that and how much fresh water does sea water flushing save?
ChallengeTertiary treatment adds phosphorus and nitrogen removal. Which environmental problem does it mainly prevent?
ChallengeA reservoir of 1 km² loses 2 m of water by evaporation. How many cubic metres is lost?
ChallengeA plant treats 20 000 m³ a day at 250 mg/L influent BOD, removes nine tenths and discharges into a river diluting tenfold. What is the river BOD?
Error redoSomeone says the water cycle never stops so fresh water can never run out and saving is pointless. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Calcium and magnesium ions	Calcium and magnesium ions stop soap lathering and form scale when heated.
2	Boiling, which removes temporary hardness, and ion exchange or distillation	Boiling only removes temporary hardness from hydrogencarbonates, while permanent hardness needs ion exchange or distillation.
3	Transpiration	Transpiration is the other source of water vapour in the cycle and is strong in forested areas.
4	Infiltration is water entering the soil while run-off flows over the surface	Paved ground allows little infiltration and much run-off, which is why it floods easily.
5	4.32 L a day and about 129.6 L a month	0.05 times 86 400 seconds is 4 320 mL or 4.32 L, and 30 days gives about 130 L.
6	45 L, and all of that 45 L of fresh water is saved	150 times 0.3 is 45, and sea water flushing returns that whole share of fresh water.
7	Eutrophication, where algae bloom and the water later runs short of oxygen	Phosphorus and nitrogen feed algae, so discharge to rivers and seas causes blooms that use up oxygen and kill fish.
8	2 000 000 m³	1 km² is 1 000 000 m², and times 2 m gives 2 000 000 m³.
9	About 2.5 mg/L	250 times 0.1 is 25, and dividing by 10 gives 2.5 mg/L.
10	The cycle is a flow while usable fresh water is a limited stock, and pollution or over use causes shortages	The cycle replenishes only so much each year, so drawing faster than replenishment or polluting the source brings shortage.

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.