AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2020: Question 2

✅ Model Answer (Level 2: 4 Marks)

1. Select a fresh water source: An appropriate source of fresh water is chosen, such as a river, stream, lake, or borehole/reservoir.
2. Filtration: The water is passed through filter beds (e.g. sand and gravel) to remove insoluble/undissolved solids.
3. Sterilisation: The filtered water is sterilised using chlorine, ozone, or ultraviolet (UV) light to destroy/kill harmful microbes and bacteria.

💡 Key Knowledge

• Potable water is water that is safe to drink; it is not pure water because it still contains dissolved salts and minerals in safe quantities.
• Fresh water contains low levels of dissolved substances.
• Filtration targets undissolved suspended solids.
• Sterilisation targets living pathogens/microorganisms.

🧠 Exam Technique: Level 2 Criteria

This is a "levels of response" question:

• To get 3–4 marks (Level 2): You must state both the action and the reason (e.g. "filter beds" + "removes solids", and "sterilise with chlorine/UV/ozone" + "kills microbes") in a logical sequence.
• 1–2 marks (Level 1): Mentioning only names of stages without explaining what they remove or do.

❌ Common Student Errors

• Saying chlorine "cleans the water" or "removes dirt" rather than killing microorganisms/microbes.
• Confusing fresh water treatment with desalination (suggesting distillation here wastes time and loses marks).
• Forgetting to name the sterilising agents (you must know chlorine, ozone, or UV light).

✅ Correct Answer (Any one)

• Distillation
• Reverse osmosis
• Also allowed: Desalination / using membranes

🧠 Exam Technique: Decoding the Clues

Notice the three clues in the stem:

• "Very little rainfall" → cannot rely on fresh surface water.
• "Long coastline" → abundance of sea water (salty water).
• "Plentiful energy" → can afford high-energy thermal or pressure methods.

✅ Correct Pairings

💡 Mnemonic & Recall Tip

Remember the biological conditions:

• Liquid = Aerobic: Air is pumped into the liquid effluent so aerobic bacteria break down organic matter.
• Sludge = Anaerobic: Dense solid sludge sinks and is sealed in tanks without oxygen for anaerobic bacteria to digest it (producing biogas/methane).

❌ Common Errors

• Swapping aerobic and anaerobic round (the most common error).
• Drawing two lines from one box on the left: the mark scheme states that an additional line from any box negates the mark for that substance.
• Choosing physical stages like screening or sedimentation (those occur before effluent and sludge are separated).

📐 Step-by-Step Calculation

1. Identify the correct numbers from 2010 row:
   Mass burned = 260 million kg
   Total mass = 1413 million kg
2. Set up percentage equation:
   (260 / 1413) × 100 [1 mark]
3. Calculate raw answer:
   = 18.400566... % [1 mark]
4. Round to 3 significant figures:
   Look at 4th figure (0): round down → 18.4 % [1 mark]

❌ Common Errors & Traps

• Wrong row: Using 1992 numbers instead of 2010 numbers.
• Significant figures: Writing 18% (2 s.f.) or 18.40% (4 s.f.) loses the final mark. The question explicitly asked for 3 significant figures.
• Rounding incorrectly: Always write down your full unrounded calculator value before rounding so examiners can award method marks even if your rounding fails.

✅ Accepted Reasons (Any one)

• The human population increased.
• More waste water / sewage was produced.
• Less untreated sewage was discharged into rivers/seas (i.e. more sewage was treated/processed due to stricter environmental laws).

🧠 Exam Technique

When asked why a waste product has increased over time in human society, the simplest, most reliable answer is almost always population increase. More people → more human waste → more sludge.

✅ Accepted Reasons (Any two)

• Increased demand for food: Greater population needs more crops grown / more farming.
• Sustainability / resource conservation: Conserves energy/resources compared to manufacturing synthetic fertilisers (Haber process).
• Landfill capacity limits: Landfill space is running out / greater awareness of negative environmental effects of landfill.
• Demand for organic farming: Increased demand for organic fertiliser as a lifestyle/consumer choice.

❌ Examiner Guidance & Ignored Points

• Do NOT just repeat the data: Stating "less was sent to landfill" is just reading the table, not explaining why.
• Ignore cost: Simply stating "it is cheaper" or "costs less" is ignored unless linked to specific resource conservation.
• Be specific: state "landfill space running out" rather than just "landfills are bad".