AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2020: Question 9

✅ Model Answer (Level 2: 4 Marks)

1. Source: Choose an appropriate source of fresh water (e.g. rivers, streams, lakes, or underground boreholes/aquifers).
2. Filtration: Pass the water through filter beds (made of sand and gravel) to remove undissolved solids.
3. Sterilisation: Sterilise the filtered water using chlorine, ozone, or ultraviolet (UV) light to destroy harmful bacteria/microbes.

🧠 Exam Technique & Level Descriptors

This is a levels-of-response question:

• Level 2 (3–4 marks): Relevant points are identified, explained in detail, and linked logically from source to sterilisation.
• Level 1 (1–2 marks): Points are just stated as isolated keywords without linking what they achieve (e.g. just writing "filter it and add chlorine").

💡 Key Definitions

• Potable water: Water that is safe to drink. It is not pure water because it still contains dissolved mineral salts.
• Pure water: 100% H₂O with no dissolved solutes, boiling at exactly 100°C.

❌ Common Errors

• Confusing filtration with sterilisation: Stating that "filtration removes bacteria" or "chlorine cleans out the dirt".
• Mentioning distillation: Distillation is not needed for fresh water in the UK because fresh water already has low levels of dissolved salts.

✅ Accepted Answers (Any ONE)

• Desalination
• Distillation (thermal desalination)
• Reverse osmosis (or use of semi-permeable membranes)

💡 Clues in the Question Stem

The question notes: very little rainfall, long coastline, and plentiful energy supplies.

• Coastline: Abundant supply of salty sea water.
• Plentiful energy: Desalination processes require huge quantities of thermal or electrical energy.

💡 How to Remember the Biology

• Liquid = Aerobic: Air is bubbled through the liquid effluent to encourage aerobic bacteria to break down organic matter.
• Sludge = Anaerobic: Sludge is dense and settles at the bottom; oxygen cannot easily reach it, so anaerobic bacteria digest it, producing biogas (methane).

❌ Mark Scheme Warning

"Additional line from a box on the left negates the mark for that box."

Draw strictly one straight line from each left-hand box. If you draw two lines from "Liquid effluent", you automatically score 0 for that part.

❌ Common Errors

• Reading 1992 data instead of 2010: Always double-check which row you are taking numbers from.
• Incorrect rounding: Writing 18% (2 sig figs) or 18.40% (4 sig figs) loses the final mark.

🧠 Error-Carried-Forward (ECF)

Even if you made an arithmetic error or picked incorrect numbers, correctly rounding your final percentage to 3 significant figures still scores the 3rd mark!

✅ Accepted Reasons (Any ONE)

• The population increased.
• More waste water was produced.
• Less untreated sewage was discharged into waterways / better sewage collection infrastructure.

🧠 Examiner Insight

More humans = more toilet and domestic water use = more sewage. "Population increase" is the simplest and safest answer to give for waste/resource trend questions.

✅ Accepted Reasons (Any TWO)

• Food demand: Increased demand for food / more farming due to an increasing population.
• Sustainability: Conserves resources / energy / more sustainable than manufacturing synthetic fertilisers (Haber process).
• Landfill constraints: Landfill space is running out / increased awareness of environmental damage from landfill.
• Organic demand: Increased consumer demand for organic fertilisers.

❌ What the Mark Scheme Rejects

• Cost: Mark scheme explicitly states: "ignore references to cost" (e.g. "it is cheaper").
• Repeating data: "Ignore less sent to landfill" – simply re-stating numbers from the table does not explain why.