Edexcel A-Level Chemistry Paper 1, June 2018: Question 1

✅ Correct Answer

Ammonium bromide (Allow names in either order, or formula NH₄Br ).

💡 Key Knowledge

• Cation test: Adding NaOH(aq) and warming produces ammonia gas ( NH₃ ), which turns damp red litmus paper blue due to its basicity. This identifies the NH₄⁺ ion.
• Anion test: Adding acidified AgNO₃(aq) producing a cream precipitate confirms the presence of bromide ions ( Br⁻ ). ( AgCl is white, AgBr is cream, AgI is yellow).

❌ Common Errors

• Writing ammonia instead of ammonium for the cation name (ammonia is the gas evolved, not the cation present in the salt).
• Writing bromine instead of bromid e for the anion name.

🧠 Exam Technique

Examiners mark this independently (1 mark for the cation, 1 mark for the anion). Read the question carefully: it asks for the name, so ensure you write out words clearly rather than just formulae, though formulae are accepted if unambiguous.

✅ Correct Answer

Add dilute aqueous ammonia to the cream precipitate and observe that it is insoluble (1 mark). Then add concentrated aqueous ammonia and observe that it is soluble / dissolves to form a colourless solution (1 mark).

💡 Key Knowledge

• AgCl dissolves in dilute ammonia.
• AgBr is insoluble in dilute ammonia, but dissolves in concentrated ammonia.
• AgI is insoluble in both dilute and concentrated ammonia.
• Alternative test: Add concentrated sulfuric acid to the solid salt; observation of brown fumes (due to Br₂ gas) also confirms bromide.

❌ Common Errors

• Forgetting to specify excess ammonia or failing to link the reagent concentration correctly to the solubility outcome.
• Stating that the precipitate dissolves in dilute ammonia (which is true for chloride, not bromide).

🧠 Exam Technique

To secure full marks in qualitative analysis descriptions, always state: 1. The exact reagent added (including concentration), and 2. The precise visible observation (e.g., "precipitate dissolves" or "remains insoluble").