A student adds a few drops of sodium hydroxide solution to four separate test tubes, each containing a solution of a different metal salt. Describe what the student would observe in each test tube if the solutions contained iron(II) ions, iron(III) ions, copper(II) ions, and calcium ions respectively, and explain what these observations indicate about transition metals and calcium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium hydroxide solution is commonly used as a reagent to identify cations in solution. Different metal ions produce characteristic precipitates when sodium hydroxide solution is added.
Model answer (4 marks)
Iron(II) ions: green precipitate forms.
Iron(III) ions: orange/brown precipitate forms.
Copper(II) ions: blue precipitate forms.
Calcium ions: white precipitate forms.
The coloured precipitates from Fe²⁺, Fe³⁺ and Cu²⁺ show that transition metals can form coloured compounds, whereas Ca²⁺, a non‑transition metal, gives a white precipitate.
Iron(III) ions: orange/brown precipitate forms.
Copper(II) ions: blue precipitate forms.
Calcium ions: white precipitate forms.
The coloured precipitates from Fe²⁺, Fe³⁺ and Cu²⁺ show that transition metals can form coloured compounds, whereas Ca²⁺, a non‑transition metal, gives a white precipitate.
Examiner tips
- Use the exact colour descriptors (green, orange/brown, blue, white).
- Mention the precipitate colour before the explanation of transition metal behaviour.
- Keep the explanation short and directly link colour to transition metal property.
- Use the phrase "coloured precipitates" to match the mark scheme.
Common mistakes
- Writing the wrong colour for a precipitate (e.g. green for Fe³⁺).
- Omitting the explanation that transition metals form coloured compounds.
- Using vague terms like "colourful" instead of the specific colours.
Mark scheme (4 marks)
- Iron(II) ions produce a green precipitate with sodium hydroxide solution
- Iron(III) ions produce an orange/brown precipitate with sodium hydroxide solution
- Copper(II) ions produce a blue precipitate with sodium hydroxide solution
- Calcium ions produce a white precipitate with sodium hydroxide solution; the coloured precipitates from iron(II), iron(III), and copper(II) show that transition metals form coloured compounds, whereas calcium (which is not a transition metal) forms a white precipitate
Key terms in this question
transition metal · sodium hydroxide
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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