Q6 — Cation identification with NaOH (3 marks)

Question: A student adds sodium hydroxide solution dropwise to three unknown salt solutions, X, Y and Z.

• X gives a blue precipitate.
• Y gives a green precipitate that turns brown on standing.
• Z gives a red-brown precipitate.

Identify the metal ion in each solution and write the ionic equation for the reaction in solution X.

Mark scheme (3 marks):

• X = Cu²⁺ (copper(II)); Y = Fe²⁺ (iron(II)); Z = Fe³⁺ (iron(III)) — all three required for 1 mark; partial credit not given (1)
• Ionic equation: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) (1)
• State symbols correct, especially (s) on the precipitate (1)
• Reject formula 'CuOH' or 'Cu(OH)₃'.