AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2024: Question 10

✅ Correct Selections

• Combustion [1 mark]
• Reversible [1 mark]

💡 Key Knowledge

• Hydrogen burns in oxygen to form water: 2H₂ + O₂ → 2H₂O . Burning with oxygen is combustion (which is highly exothermic).
• The text explicitly notes: "Water is decomposed into hydrogen gas and oxygen gas using electricity." Because the products can react to reform the reactants, it is a reversible process.

❌ Common Errors

• Ticking Endothermic: Combustion releases heat into the surroundings, meaning it is exothermic, not endothermic.
• Ticking Alloying or Corrosion: Hydrogen and oxygen are non-metals, so they do not make alloys or undergo rusting/corrosion.

🧠 Exam Technique

Always read the introductory sentences carefully. The exam paper deliberately gave both directions of the reaction: hydrogen + oxygen forming water, and electricity decomposing water back into hydrogen and oxygen. That was your clue for reversible!

✅ Correct Answer

Water molecules break down and reform at the same rate. [1 mark]

💡 Key Knowledge

• The symbol ⇌ indicates a reversible reaction.
• At chemical equilibrium in a closed system:
  1. The forward and reverse reactions happen at the exact same rate.
  2. The concentrations of reactants and products remain constant.

❌ Common Misconceptions

• Thinking equilibrium means "the reaction has stopped" (it is dynamic — both reactions are still happening!).
• Confusing equal rates with equal amounts. Equilibrium means equal rates, not necessarily 50% reactants and 50% products.

✅ Model Answer Structure

Process A: Producing Potable Water (from Ground Water)

• Filtration: Water is passed through filter beds (e.g. sand and gravel) to remove insoluble suspended solids.
• Sterilisation: Water is treated with chlorine, ozone, or exposed to ultraviolet (UV) light to destroy harmful microbes/bacteria and make it safe to drink.

Process B: Waste Water Treatment (Sewage)

• Screening: Sewage passes through metal grids/screens to remove large solid objects and grit.
• Sedimentation: Sewage sits in settling tanks; heavier solids sink to form sludge, while the liquid layer floating above forms effluent.
• Biological Treatment:
  • The sludge is digested by bacteria anaerobically (without oxygen).
  • The effluent is broken down by bacteria aerobically (with oxygen).

🧠 Exam Technique & Tips for Full Marks

• Notice the Level 3 hurdle: The mark scheme states "access to Level 3 requires reference to both potable water production and waste water treatment." If you only write about one, the maximum you can score is 4 marks!
• State the purpose of each step: Don't just list the name of a method; give the reason (e.g. "filter beds... to remove solids", "chlorine... to kill microbes").
• Follow the question guidance: The question specifically warns: "Do not refer to use of water in the home." Writing about baths, dishwashing, or toilets wastes valuable exam time and earns zero marks.

❌ Common Errors in Water Questions

• Mixing up "Pure" and "Potable": Potable water is safe to drink, but it is not pure water because it still contains dissolved mineral salts. Pure water contains only H₂O molecules.
• Confusing aerobic and anaerobic digestion:
  • Sludge → Anaerobic (produces biogas/fertiliser).
  • Effluent → Aerobic (air bubbled through).
• Saying "filter out bacteria": Filter beds remove sand, grit, and insoluble dirt. Microorganisms are killed in the sterilisation stage using sterilising agents, not filtered out.

💡 Summary Table: Key Terms