AQA A-Level Chemistry AS Paper 1, November 2021: Question 19

✅ Correct Answer: D

NaI(aq) + Br₂(aq)

Bromine is more reactive than iodine and will displace iodide ions from solution, forming aqueous iodine ( I₂ ), which appears as a characteristic dark brown solution (or purple/violet in an organic solvent).

💡 Key Knowledge

• Halogen reactivity trend: Reactivity decreases down Group 7 (F₂ > Cl₂ > Br₂ > I₂).
• Displacement rule: A more reactive halogen will displace a less reactive halide ion from its salt solution.
• Halogen colours in aqueous solution:
  • Chlorine = pale green / colourless
  • Bromine = orange / yellow
  • Iodine = brown / dark brown

🧠 Exam Technique

Scan the multiple-choice options methodically using your reactivity series:

• Check if the free halogen (element) is more reactive than the halide in the compound.
• Eliminate options where a less reactive halogen tries to displace a more reactive halide (e.g., Options A, B, C).
• Match the resulting free halogen to its known aqueous colour to confirm the final visual observation.

❌ Common Errors

• Confusing the reactivity trend of halogens with that of Group 2 metals (where reactivity increases down the group).
• Assuming a reaction occurs simply because two solutions are mixed, ignoring the underlying redox displacement rules.
• Mixing up the colours of aqueous bromine and aqueous iodine.