AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2022: Question 5

12 marks · Standard Demand difficulty · Short Answer

Describe sewage treatment, analyze desalination and filtration of ground water, explain the everyday meaning of 'pure', and describe the chemical test for chlorine.

Practise this question

Question

A GCSE chemistry exam question about water treatment. Question 5.1 asks to describe how sewage is treated to remove organic matter. A table compares sea water and ground water treatment, showing ion concentrations before and after Process 1 (reverse osmosis for sea water, filtration for ground water) and Process 2 (adding ozone for sea water, UV light for ground water). Question 5.2 asks to identify the correct concentrations of sodium and chloride ions after reverse osmosis. Question 5.3 asks why filtration doesn't change ion concentrations in ground water. Question 5.4 asks why ground water requires Process 2. Question 5.5 asks what a company means by 'pure' water. Question 5.6 asks for the test and result for chlorine gas.
Question text

05 This question is about water.

05.1 Sewage is waste water.

Sewage contains organic matter.

Describe how sewage is treated to remove organic matter.

[4 marks]

Sea water and ground water are treated to make them potable.

Table 5 shows information about the composition and treatment of sea water and of

ground water.

Table 5

Sea water Ground water

Concentration of sodium ions and Na+ : 0.5 mol/dm3 Na+ : 0.001 mol/dm3

chloride ions before Process 1 Cl− : 0.5 mol/dm3 Cl− : 0.001 mol/dm3

Process 1 Reverse osmosis Filtration

Concentration of sodium ions and Na+ : 0.001 mol/dm3

X − 3

chloride ions after Process 1 Cl : 0.001 mol/dm

Process 2 Add ozone Expose to ultraviolet light

05.2 Sea water is desalinated during Process 1.

Which pair of concentrations could represent X in Table 5?

[1 mark]

Tick ( ) one box.

Na+ : 0.003 mol/dm3 Cl− : 0.003 mol/dm3

Na+ : 0.003 mol/dm3 Cl− : 0.5 mol/dm3

Na+ : 0.5 mol/dm3 Cl− : 0.003 mol/dm3

Na+ : 0.5 mol/dm3 Cl− : 0.5 mol/dm3

05.3 Explain why the concentrations of sodium ions and of chloride ions in the ground

water in Table 5 are unchanged by Process 1.

[2 marks]

05.4 Explain why the ground water in Table 5 requires Process 2 before the water is

safe to drink.

[2 marks]

05.5 After treatment the ground water in Table 5 is sold by a company as pure water.

The ground water in Table 5 is not chemically pure because the water contains

sodium ions and chloride ions.

Suggest what the company means by ‘pure’.

[1 mark]

05.6 Chlorine is also used to treat some ground water.

Describe the test for chlorine gas.

Give the result of the test.

[2 marks]

Test

Result

Mark scheme

Show the mark scheme The mark scheme for Question 5. 5.1 awards 4 marks for screening/grit removal, sedimentation, anaerobic digestion of sludge, and aerobic biological treatment of effluent. 5.2 awards 1 mark for selecting the lowest concentration pair (0.003 mol/dm3). 5.3 awards 2 marks for explaining that ions pass through the filter because they are in solution or smaller than pores. 5.4 awards 2 marks for explaining that ground water contains harmful microbes and Process 2 sterilizes it. 5.5 awards 1 mark for 'unadulterated' or 'nothing added'. 5.6 awards 2 marks for using damp litmus paper and showing it is bleached or turns white.

Question 5

AO /

Question Answers Extra information Mark

Spec. Ref.

05.1 allow a description of each AO1

process 4.10.1.3

screening allow filtering to remove (large) 1

or solids

grit removal

sedimentation (to produce 1

sewage sludge and effluent)

anaerobic digestion of (solid 1

sewage) sludge

aerobic biological treatment of allow aerobic digestion of 1

(liquid) effluent effluent

AO /

Spec. Ref.

Na+ : 0.003 mol/dm3 Cl− : 0.003 mol/dm3 1 AO3

05.2

4.10.1.2

AO /

Spec. Ref.

05.3 the ions pass through the filter allow the ions are not trapped / 1 AO3

removed by the filter 4.1.1.2

4.10.1.2

(because) the ions are in allow (because) the ions are 1

solution smaller than the filter pores

AO /

Spec. Ref.

05.4 (the ground water) contains 1 AO3

microbes which are harmful (to

health)

AO1

(so) the water is sterilised 1

or

(so) the microbes are destroyed 4.10.1.2

AO /

Spec. Ref.

05.5 (the water is) unadulterated allow nothing is added (to the 1 AO2

or water) 4.8.1.1

(the water is) in its natural state 4.10.1.2

allow (the water) contains no

microbes

AO /

Spec. Ref.

MP2 is dependent upon MP1

AO1

05.6

(use) damp litmus paper 1 4.8.2.4

(the paper) is bleached ignore paper turns red 1

or

(the paper) turns white

Total Question 5 12

How to answer it

Potable Water & Water Treatment

What this question tests

This question assesses your understanding of how we obtain potable (safe to drink) water from different sources. You need to recall the stages of sewage treatment, interpret data on desalination and filtration, explain the purpose of sterilisation, distinguish between chemical and everyday definitions of purity, and recall the chemical test for chlorine gas.

Question 05.1

Sewage Treatment Process

Describe how sewage is treated to remove organic matter. [4 marks]

Correct Answer

  1. Screening or grit removal to remove large solids. [1 mark]
  2. Sedimentation to produce sewage sludge and effluent. [1 mark]
  3. Anaerobic digestion of the solid sewage sludge. [1 mark]
  4. Aerobic biological treatment of the liquid effluent. [1 mark]

Key Knowledge

  • Sludge vs. Effluent: Sedimentation separates sewage into solid sludge (which sinks) and liquid effluent (on top).
  • Aerobic vs. Anaerobic:
    • Sludge = Anaerobic (no oxygen).
    • Effluent = Aerobic (needs oxygen).

Exam Technique

Write your answer in chronological order. Use the correct biological terms: aerobic and anaerobic. Examiners look for these specific keywords paired with the correct sewage fraction (sludge or effluent).

Common Errors

Students often swap the treatments: they incorrectly state that sludge is treated aerobically and effluent anaerobically. Remember: Sludge is Sealed (Anaerobic).

Question 05.2

Desalination & Concentration Changes

Which pair of concentrations could represent X in Table 5? [1 mark]

Correct Answer

Na⁺: 0.003 mol/dm³     Cl⁻: 0.003 mol/dm³ ✓

This is the first option in the list.

Key Knowledge

Reverse osmosis is a method of desalination. It removes the vast majority of dissolved salts (sodium and chloride ions) from sea water, so the concentration of both ions must decrease significantly from their starting value of 0.5 mol/dm³.

Question 05.3

Filtration of Ground Water

Explain why the concentrations of sodium ions and of chloride ions in the ground water in Table 5 are unchanged by Process 1 (Filtration). [2 marks]

Correct Answer

  • The ions pass through the filter / are not trapped by the filter. [1 mark]
  • Because the ions are in solution / are smaller than the filter pores. [1 mark]

Exam Technique

An "explain" question requires a cause-and-effect link.
Cause: The ions are dissolved in solution (extremely small).
Effect: They easily pass through the gaps in the filter paper.

Question 05.4

Sterilisation of Water

Explain why the ground water in Table 5 requires Process 2 (Expose to ultraviolet light) before the water is safe to drink. [2 marks]

Correct Answer

  • The ground water contains microbes/bacteria which are harmful to health. [1 mark]
  • UV light sterilises the water / destroys the microbes. [1 mark]

Key Knowledge

Water is sterilised to kill pathogens (harmful microorganisms). The three main sterilising agents used for potable water are:
• Chlorine gas
• Ozone
• Ultraviolet (UV) light

Question 05.5

The Definition of "Pure"

Suggest what the company means by 'pure'. [1 mark]

Correct Answer

The water is unadulterated / in its natural state (or nothing has been added to it / it contains no microbes). [1 mark]

Common Errors

Do not say "it only contains water molecules". That is the chemical definition of pure. In everyday marketing, "pure" means natural and unprocessed.

Question 05.6

Chemical Test for Chlorine Gas

Describe the test for chlorine gas. Give the result of the test. [2 marks]

Correct Answer

Test: Use damp litmus paper. [1 mark]

Result: The paper is bleached / turns white. [1 mark]

Exam Technique

The word "damp" is essential to secure the first mark. Dry litmus paper will not react with chlorine gas.
Note: The second mark is dependent on the first!

Topics

Chemistry · C10: Using Resources · C8: Chemical Analysis

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