Metal Hydroxide Tests
Practise Metal Hydroxide Tests for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) chemistry topic: add sodium hydroxide solution and identify a metal ion from the colour of the precipitate formed.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Some metal ions can be identified by adding sodium hydroxide solution, which forms coloured insoluble metal hydroxide precipitates. This is a Triple-only topic.
Three ions give distinctive colours: copper(II) forms a blue precipitate, iron(II) forms a green precipitate, and iron(III) forms a brown precipitate.
Four ions give a white precipitate: calcium, magnesium, aluminium and zinc. Aluminium is distinguished by adding excess sodium hydroxide, in which its white precipitate dissolves again to form a colourless solution. The others remain as white precipitates.
Revision notes
The coloured precipitates
Add sodium hydroxide solution to the sample.
Copper(II) gives a blue precipitate. Iron(II) gives a green precipitate. Iron(III) gives a brown precipitate. These three are distinctive and easily identified.
The white precipitates
Calcium, magnesium, aluminium and zinc ions all give white precipitates.
Because four ions give the same result, further testing is needed to distinguish them.
Distinguishing aluminium
Add excess sodium hydroxide solution.
The aluminium hydroxide precipitate dissolves again, forming a colourless solution. The precipitates of calcium, magnesium and zinc do not dissolve in excess. This is the standard way to identify aluminium.
Key points
- Sodium hydroxide forms metal hydroxide precipitates.
- Copper(II) gives a blue precipitate.
- Iron(II) gives a green precipitate.
- Iron(III) gives a brown precipitate.
- Calcium, magnesium, aluminium and zinc give white precipitates.
- Aluminium hydroxide dissolves in excess sodium hydroxide.
Worked examples
Example 1
State the colour of the precipitate formed when sodium hydroxide is added to a solution containing iron(III) ions. [1 mark]
Model answer
Brown (1 mark).
Example 2
Explain how you would distinguish aluminium ions from magnesium ions. [3 marks]
Model answer
Add sodium hydroxide solution to both; each forms a white precipitate (1 mark). Then add excess sodium hydroxide (1 mark). The aluminium hydroxide precipitate dissolves to form a colourless solution, whereas the magnesium hydroxide precipitate does not (1 mark).
Example 3
State the colours of the precipitates formed by copper(II) and iron(II) ions. [2 marks]
Model answer
Copper(II) gives a blue precipitate (1 mark). Iron(II) gives a green precipitate (1 mark).
Common mistakes
Confusing iron(II) and iron(III) colours.
Iron(II) is green; iron(III) is brown.
Saying all white precipitates are the same ion.
Four different ions give white precipitates, so further testing is needed.
Forgetting the excess sodium hydroxide test.
It is the only way to identify aluminium among the white precipitates.
Saying the precipitate is a metal rather than a metal hydroxide.
The precipitate is the insoluble metal hydroxide.
Exam tips
- Learn the three coloured precipitates precisely.
- Remember four ions give white precipitates.
- Use excess sodium hydroxide to identify aluminium.
- Remember this is a Triple-only topic.
Key terms
- Precipitate
- An insoluble solid formed in a solution.
- Metal hydroxide
- The insoluble compound formed with sodium hydroxide.
- Excess
- More than enough, used to dissolve aluminium hydroxide.
- Iron(II)
- An iron ion with a 2+ charge, giving a green precipitate.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.