Neco 2022 Chemistry Question 14

A sample solution contains an unknown mixture of the metal ions Fe³⁺, Cu²⁺, Ni²⁺ and Zn²⁺. The following qualitative tests are carried out sequentially:\n\n1. Addition of dilute HCl produces a white precipitate that dissolves on heating.\n2. To the filtrate, excess aqueous NH₃ is added, giving a deep‑blue solution.\n3. After adjusting the solution to pH 9, dimethylglyoxime reagent is added, producing a bright‑red precipitate.\n4. In a separate experiment, the original solution is first treated with excess KCN; the solution becomes colourless and no precipitate forms when H₂S is subsequently added in acidic medium.\n\nBased on these observations, identify all metal ions present in the original mixture.