AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2022: Question 2

✅ Correct answers

• Ground water is easier to obtain than waste water.
• It needs fewer treatment processes and takes less time.
• It is usually filtered through filter beds to remove insoluble particles.
• It is then sterilised using chlorine, ozone or UV light to kill bacteria.
• Waste water is more difficult to obtain as potable water.
• It needs more processes: screening and grit removal, sedimentation, aerobic biological treatment, then sterilisation.

💡 Key knowledge

• Potable water means water that is safe to drink.
• Ground water is already relatively clean, so fewer steps are needed.
• Waste water/sewage contains larger particles, organic matter and bacteria, so it must be cleaned more thoroughly.
• Common final sterilisation methods: chlorine, ozone or UV light.

🧠 Exam technique

• Use comparative language: “easier than”, “more than”, “fewer than”, “less time”.
• To reach the top level, identify relevant features and make the differences clear.
• Best answers often describe the process in the correct order for each source.
• Because this is 6 marks, aim for at least 4–6 correct comparative points.

❌ Common errors

• Describing only one source and forgetting to compare it with the other.
• Saying waste water needs only filtration — it needs several stages first.
• Confusing filter beds with screening.
• Forgetting that both sources are sterilised at the end.
• Writing “cleaned” without naming a proper process.

✅ Correct answer

Distillation

💡 Key knowledge

Salty water contains dissolved salt. To make potable water, you must separate the water from the dissolved substances. Filtration will not remove dissolved salt.

🧠 Exam technique

For a 1-mark multiple choice question, only the exact correct process gets the mark. If you are unsure, remember: dissolved salt + water = distillation.

❌ Common errors

• Filtration only removes insoluble solids.
• Electrolysis breaks compounds apart using electricity, not a method for making drinking water here.
• Sterilisation kills microorganisms but does not remove salt.

✅ Correct answer

Concentration of sodium chloride = 16 g/dm³

🧠 Exam technique

• Use the correct unit conversion: cm³ to dm³.
• Show your working clearly to earn method marks.
• Keep the answer to a sensible number of significant figures; here 16 is acceptable.

❌ Common calculation traps

• Using 150 instead of 0.15 dm³.
• Dividing the wrong way round.
• Forgetting the unit g/dm³.
• Writing 0.016 and forgetting to convert to dm³.
• Rounding too early before the final step.

💡 Top-level response features

• Clear comparison between sewage and ground water.
• Correct treatment sequence for waste water.
• Recognition that both need sterilisation.
• Accurate distillation answer for salty water.
• Correct unit conversion in the calculation.

❌ Where students lost marks

• Not making the comparison explicit in part (a).
• Giving general statements like “it is purified” without naming stages.
• Using the wrong process for salty water.
• Forgetting to convert cm³ to dm³.

🧠 Best way to secure full marks

For comparison questions, write in pairs:

Ground water... / Waste water...

For calculations, always write:

convert → substitute → calculate → include units