RP11: Carry out simple test-tube reactions to identify transition metal ions in aqueous solution

Describe how you would identify an unknown transition metal ion in aqueous solution. (5 marks)

1. Note the colour of the original solution, which already narrows the possibilities.

2. Add sodium hydroxide dropwise and record the colour of any precipitate.

3. Continue adding until in excess, and record whether the precipitate dissolves.

4. Repeat with a fresh sample, adding ammonia dropwise and then in excess.

5. Repeat with a fresh sample and add sodium carbonate, noting whether there is effervescence.

State the colours of the precipitates formed with sodium hydroxide for each of the four ions. (4 marks)

• Fe²⁺: green, darkening to brown on standing.

• Cu²⁺: blue.

• Fe³⁺: brown.

• Al³⁺: white.

Explain how excess sodium hydroxide distinguishes Al³⁺ from Fe³⁺. (2 marks)

• Both give a precipitate at first, white for Al³⁺ and brown for Fe³⁺.

• Only the aluminium hydroxide dissolves in excess, as it is amphoteric, giving a colourless solution.

Explain how excess ammonia distinguishes Cu²⁺ from the other ions. (2 marks)

• All four give a precipitate with dilute ammonia.

• Only the copper precipitate dissolves in excess, giving a deep blue solution as [Cu(NH₃)₄(H₂O)₂]²⁺ forms.

Explain how sodium carbonate distinguishes a 2+ ion from a 3+ ion. (2 marks)

• A 2+ ion gives a carbonate precipitate with no effervescence.

• A 3+ ion is acidic enough to release carbon dioxide, so effervescence is seen along with a hydroxide precipitate.

Explain why a fresh sample and a clean test tube are used for each test. (3 marks)

• A reagent left from an earlier test would react and give a misleading result.

• For example hydroxide left in the tube would precipitate the metal before the next reagent was added.

• Using a fresh portion each time also means an earlier reagent cannot mask a later observation.

State the safety precautions needed for this practical. (3 marks)

• Wear eye protection, as sodium hydroxide is corrosive.

• Use dilute ammonia in a well ventilated room, as the vapour is an irritant.

• Many transition metal salts are harmful, so avoid skin contact and wash hands afterwards.