A student has four separate solutions, each containing one of the following ions: Fe²⁺, Fe³⁺, Cu²⁺, or Zn²⁺. The student adds an excess of sodium hydroxide solution to each solution and observes the precipitate formed. Describe the observations the student would make for each of the four solutions when excess sodium hydroxide solution is added.

Pearson Edexcel International GCSE Chemistry (4CH1) — 2.5 Tests for ions and gases · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Each solution is colourless except the copper(II) solution, which is pale blue. The student adds excess sodium hydroxide solution to each solution in turn.

Model answer (4 marks)

Fe²⁺: green precipitate
Fe³⁺: red‑brown (or orange‑brown) precipitate
Cu²⁺: blue precipitate
Zn²⁺: white precipitate that dissolves in excess NaOH, giving a colourless solution

Examiner tips

  • Use the exact colour terms (green, red‑brown, blue, white) and note the dissolution for Zn²⁺
  • Mention ‘excess’ NaOH to show the dissolution step
  • Keep each observation brief – one line per ion

Common mistakes

  • Confusing the colours of Fe²⁺ and Fe³⁺; writing green for Fe³⁺ or red‑brown for Fe²⁺
  • Failing to state that the Zn²⁺ precipitate dissolves, leaving a colourless solution

Mark scheme (4 marks)

  1. Fe²⁺ gives a green precipitate with sodium hydroxide solution
  2. Fe³⁺ gives a red-brown (or orange-brown) precipitate with sodium hydroxide solution
  3. Cu²⁺ gives a blue precipitate with sodium hydroxide solution
  4. Zn²⁺ gives a white precipitate that dissolves in excess sodium hydroxide solution (forming a colourless solution)

Key terms in this question

precipitate · sodium hydroxide solution · excess · Fe²⁺ · Fe³⁺ · Cu²⁺ · Zn²⁺

Related

More Tests for ions and gases questions

▶ Try answering this question with AI marking (free) →