AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2024: Question 3
9 marks · Standard Demand difficulty · Extended Answer
Identify the characteristics of the reaction between hydrogen and oxygen, define dynamic equilibrium for water dissociation, and explain the processes for producing potable water and treating sewage.
Practise this questionQuestion
Question text
03 This question is about water.
03.1 Hydrogen gas reacts with oxygen gas to produce water.
Water is decomposed into hydrogen gas and oxygen gas using electricity.
Which two words describe the reaction between hydrogen gas and oxygen gas?
[2 marks]
Tick ( ) two es.
Alloying
Combustion
Corrosion
Endothermic
Reversible
03.2 Water molecules break down into hydrogen ions and hydroxide ions.
The equation for the reaction is:
H O ⇌ H+ + OH–
Which sentence describes this reaction at equilibrium?
[1 mark]
Tick ( ) one box.
Water molecules break down at a higher rate than they reform.
Water molecules break down and reform at the same rate.
Water molecules break down at a lower rate than they reform.10
03.3 Water collected from rivers is used in the home for drinking and flushing toilets.
Water used in the home must be potable.
*09* Potable water is safe to drink.
Waste water produced after use in the home is called sewage.
Figure 3 shows how water is collected from rivers and returned to rivers after use.
Figure 3
Explain what happens to water in Process A and in Process B in Figure 3.
Do not refer to use of water in the home.
[6 marks]
Mark scheme
Show the mark scheme
Question 3
AO /
Question Answers Extra information Mark
Spec. Ref.
03.1 combustion 1 AO2
4.6.2.1
reversible 1
AO /
Spec. Ref.
03.2 water molecules break down 1 AO2
and reform at the same rate – – – 4.6.2.3
AO /
Question Answers Mark
Spec. Ref.
03.3 Level 3: Relevant points (reasons / causes) are identified, given in 5–6 AO1
detail and logically linked to form a clear account. 4.10.1.2
4.10.1.3
Level 2: Relevant points (reasons / causes) are identified, and 3–4
there are attempts at logical linking. The resulting account is not
fully clear.
Level 1: Points are identified and stated simply, but their relevance 1–2
is not clear and there is no attempt at logical linking.
No relevant content 0
Indicative content
Potable water production
• pass water through filter beds
• to remove solids
• use chlorine / ozone / UV light
• to sterilise water
• to destroy microbes
Waste water treatment
• screening
• using a metal grid
• to remove solids
• to remove grit
• sedimentation
• to produce sewage sludge and effluent
• anaerobic digestion of sewage sludge 13
• aerobic biological treatment of effluent
access to Level 3 requires reference to both potable water
production and waste water treatment.
Total Question 3 9
How to answer it
Water: Reactions, Equilibrium, and Treatment
What this question tests
This question assesses your understanding of chemical reaction types, the definition of dynamic equilibrium, and the industrial processes used to produce potable (drinkable) water and treat wastewater (sewage).
Describing Chemical Reactions
Identifying reaction types from a description
Correct Answers
- Combustion
- Reversible
Key Knowledge
- Combustion: Any reaction where a substance reacts with oxygen. Hydrogen "burns" in oxygen to form water.
- Reversible: The question states water is formed from H₂ and O₂, but can also be "decomposed" back into them. This means the reaction can go in both directions.
Common Errors
Students often mistake the reaction for Endothermic. However, burning hydrogen releases a huge amount of energy (it is highly Exothermic). Alloying and Corrosion refer to metals, not gases forming water.
Understanding Equilibrium
Defining the state of a reversible reaction
Correct Answer
Water molecules break down and reform at the same rate.
Exam Technique
Whenever you see the word equilibrium in a GCSE chemistry paper, look for the phrase "same rate" or "equal rate". Equilibrium does NOT mean the amounts of products and reactants are equal; it means the speed of the forward and backward reactions is the same.
Water Treatment Processes
Comparing Potable Water vs. Waste Water Treatment
Process A: Potable Water
Goal: Make water safe to drink.
- Filtration: Pass water through filter beds (sand/gravel) to remove insoluble solids.
- Sterilisation: Add chlorine, ozone, or use UV light to kill/destroy microbes (bacteria).
Process B: Waste Water
Goal: Remove contaminants before returning to rivers.
- Screening: Use a metal grid to remove large objects and grit.
- Sedimentation: Allow the water to sit so sludge settles at the bottom and effluent floats on top.
- Treatment:
- Anaerobic digestion of sludge.
- Aerobic biological treatment of effluent.
How to score 6 Marks (Level 3)
To reach the top marks, you must:
- Mention BOTH Process A and Process B.
- Link the action to the reason (e.g., "Chlorine is added to kill bacteria").
- Use the diagram! The diagram shows a "metal grid" and "sludge"—make sure these terms appear in your answer.
Common Misconceptions
Do not confuse pure water with potable water. Pure water contains 100% H₂O molecules, while potable water is safe to drink but still contains dissolved minerals. Also, remember that filtration does not kill bacteria—that is what sterilisation is for!
What to visualize
If asked to draw these processes:
- Filter Bed: Layers of different sized rocks and sand.
- Sedimentation Tank: A large tank where heavy "sludge" is shown at the bottom and clearer "effluent" is at the top.
- Screening: A simple vertical bar chart representing a metal grid catching large debris like twigs or plastic.
Topics
Chemistry · C6: The Rate and Extent of Chemical Change · C10: Using Resources
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.