RP 11: Identifying transition metal ions

Hazards

Method

1. Read the solutions firstTake ~1 cm depth of each unknown: Cu²⁺ is blue, Fe²⁺ pale green, Fe³⁺ yellow-brown. The solution colour is the first row of your evidence table.
2. NaOH dropwise, then in excessCu²⁺: blue precipitate. Fe²⁺: green precipitate that darkens brown at the surface on standing — air oxidising Fe(II) to Fe(III). Fe³⁺: brown precipitate. None of the three redissolve in excess NaOH.
3. NH₃ dropwise, then in excessAll three give the same precipitates at first — but only Cu²⁺ redissolves in excess ammonia, forming the deep royal-blue [Cu(NH₃)₄(H₂O)₂]²⁺ solution. That colour is the single most quotable observation in the practical.
4. The carbonate cross-checkWith Na₂CO₃(aq): the 2+ ions give simple precipitates, but Fe³⁺ is acidic enough to decompose carbonate — brown precipitate WITH effervescence of CO₂.
5. Record like an examinerEvery cell of your table is colour + state: 'blue precipitate', 'green solution', 'effervescence'. A grid of ion vs reagent with an observation in every box is exactly the mark-scheme format.

Mistakes examiners see every year

What to expect

Result grid (AQA 3.2.6): Cu²⁺ — blue ppt, insoluble in excess NaOH, deep blue solution in excess NH₃; Fe²⁺ — green ppt darkening on standing; Fe³⁺ — brown ppt, effervescence with carbonate.