AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2022: Question 7

10 marks · Standard Demand difficulty · Extended Answer

Compare the processes for obtaining potable water from different sources and calculate the concentration of a sodium chloride solution.

Practise this question

Question

The image shows three parts of a chemistry exam question. Question 07.1 is a 6-mark extended response asking to compare obtaining potable water from waste water (sewage) and ground water (fresh water). Question 07.2 is a 1-mark multiple-choice question asking for the process used to produce potable water from salty water, with options: Distillation, Electrolysis, Filtration, and Sterilisation. Question 07.3 is a 3-mark calculation asking for the concentration of sodium chloride in g/dm³ given 2.40 g in 150 cm³ of salty water.
Question text

07.1 Water that is safe to drink is called potable water.

Compare how easily potable water can be obtained from:

• waste water (sewage)

• ground water (fresh water).

[6 marks]

A scientist produced potable water from 150 cm3 of salty water.

07.2 Which process can be used to produce potable water from salty water?

[1 mark]

Tick ( ) one box.

Distillation

Electrolysis

Filtration

Sterilisation

07.3 The salty water contains sodium chloride.

The scientist collected 2.40 g of sodium chloride from 150 cm3 of salty water.

Calculate the concentration of sodium chloride in grams per dm3

[3 marks]

Concentration of sodium chloride = g/dm3

Mark scheme

Show the mark scheme The mark scheme for question 7. For 07.1, it uses a level-based marking system (1-6 marks) with indicative content comparing ground water (easier, fewer processes) to waste water (harder, includes screening, sedimentation, aerobic and anaerobic treatments). For 07.2, the answer is 'distillation'. For 07.3, it shows the calculation: 150 cm³ is converted to 0.15 dm³, then 2.40 g is divided by 0.15 dm³ to give a final answer of 16 g/dm³.

Question 7

AO /

Question Answers Mark

Spec. Ref.

07.1 Level 2: Scientifically relevant features are identified, the way(s) in 4–6 AO1

which they are similar / different is made clear and (where 5.10.1.3

appropriate) the magnitude of the similarity / difference is noted.

Level 1: Relevant features are identified and differences noted. 1–3

No relevant content 0

Indicative content

ground water waste water

easier to obtain more difficult to obtain

fewer processes more processes

takes less time takes more time

filtered through filter beds screening and grit removal

to remove insoluble particles to remove large particles

sedimentation

to produce sewage sludge and

effluent

aerobic biological treatment of

effluent

to reduce solid waste

sterilised and then sterilised

using chlorine, ozone or uv using chlorine, ozone or uv

light light

to kill bacteria to kill bacteria

sludge is anaerobically

digested

by specific bacteria

to remove organic matter

AO /

Question Answers Extra information Mark

Spec. Ref.

07.2 distillation 1 AO1

5.10.1.2

AO /

Spec. Ref.

07.3 (conversion) 1 AO2

150 3 5.3.2.5

( =) 0.15 (dm )

1000

(concentration =) allow correct use of incorrect / 1

2.40 no conversion

0.15

= 16 (g/dm3)

OR

(conversion)

1000

(1)

= 6.67 (1)

(6.67 × 2.4)

= 16 (g/dm3) (1)

OR

(concentration =)

2.4

(1)

= 0.016 (1)

(conversion)

(0.016 × 1000)

= 16 (g/dm3) (1)

Total Question 7 10

How to answer it

Potable Water: Sources, Treatment, and Concentration

What this question tests

This question assesses your understanding of potable water (safe drinking water). You need to compare the industrial processes used to treat ground water versus waste water, identify methods for desalination, and perform a concentration calculation involving unit conversions.

Part (07.1)

Comparing Ground Water and Waste Water

6 Marks - Extended Writing

💡 Key Knowledge

  • Ground Water: Found in rocks/aquifers. Treatment is simple: 1. Filter through sand/gravel beds. 2. Sterilise.
  • Waste Water: Contains sewage and organic matter. Treatment is complex: 1. Screening (large bits). 2. Sedimentation (sludge and effluent). 3. Aerobic biological treatment.
  • Sterilising agents: Chlorine, Ozone, or UV light.

🧠 Exam Technique

This is a Level of Response question. To get 5-6 marks, you must compare both sources and explain why one is easier than the other.

Use comparative words: "In contrast," "However," "Whereas," or "More/Less."

✅ Model Answer Structure

Ground water is easier and faster to treat because it has fewer stages. It only requires filtering through filter beds to remove insoluble solids and then sterilising to kill bacteria.

In contrast, waste water is more difficult and takes more time. It requires screening to remove large objects, sedimentation to produce sewage sludge and effluent, and aerobic biological treatment to reduce organic matter. Like ground water, it must also be sterilised at the end using chlorine, ozone, or UV light.

Part (07.2)

Desalination

1 Mark

✅ Correct Answer

Distillation

🧠 Examiner Insight

Students often confuse filtration with distillation. Filtration only removes insoluble solids (like sand). Salt is dissolved, so you must use distillation or reverse osmosis to remove it.

Part (07.3)

Concentration Calculation

3 Marks

📐 Step-by-Step Calculation

The Goal: Find concentration in g/dm³.

  1. Step 1: Convert Volume to dm³
    There are 1000 cm³ in 1 dm³.
    150 / 1000 = 0.15 dm³ (1 Mark)
  2. Step 2: Use the Concentration Formula
    Concentration = Mass / Volume
    Concentration = 2.40 g / 0.15 dm³ (1 Mark)
  3. Step 3: Final Answer
    16 g/dm³ (1 Mark)

❌ Common Errors

  • The "Divide by 100" Trap: Many students divide by 100 instead of 1000 when converting cm³ to dm³.
  • Wrong Order: Dividing volume by mass (0.15 / 2.40) instead of mass by volume.
  • Ignoring Units: Always check if the question asks for g/dm³ or mol/dm³. This one asks for grams.

🧠 Top Tip

If you get the conversion wrong but use your wrong number correctly in the next step, you can still get 2 out of 3 marks (Error Carried Forward). Always show your working!

Total Marks Available: 10 | AQA GCSE Combined Science: Trilogy

Topics

Chemistry · C10: Using Resources · C3: Quantitative Chemistry

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.