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Analytical Chemistry Techniques
Qualitative tests for cations and gases
- 3 things to know
- A2 9
Overview
Precipitates with sodium hydroxide and ammonia, the ammonium test, tests for hydrogen, oxygen, carbon dioxide and ammonia, sulfate, carbonate and nitrate tests, unknowns, limitations and modern instruments.
What you need to know
Answer each one before you reveal it.
Where is qualitative analysis used in industry and forensic science? Give three examples.
What colours of precipitate form when sodium hydroxide is added to Cu²⁺, Fe²⁺ and Fe³⁺ ions?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is qualitative analysis?
Model answer
Qualitative analysis identifies which ions or substances are present in a sample, without measuring how much. A series of standard tests is carried out in order.
- 2
Where is qualitative analysis used in industry and forensic science? Give three examples.
Model answer
An environmental chemist identifies a pollutant in a river. A forensic chemist identifies an unknown powder, and a quality control chemist checks a raw material is the right compound.
- 3
What colours of precipitate form when sodium hydroxide is added to Cu²⁺, Fe²⁺ and Fe³⁺ ions?
Model answer
Copper(II) gives a blue precipitate and iron(II) a green precipitate. Iron(III) gives an orange-brown precipitate.
- 4
Write the ionic equation for copper(II) ions reacting with sodium hydroxide.
Model answer
Cu²⁺ + 2OH⁻ → Cu(OH)₂. The blue solid is copper(II) hydroxide.
- 5
How can sodium hydroxide distinguish aluminium ions from calcium and magnesium ions?
Model answer
All three give a white precipitate with a little sodium hydroxide. Only aluminium hydroxide dissolves again when excess sodium hydroxide is added.
- 6
How does ammonia solution distinguish copper(II) ions?
Model answer
A little ammonia solution gives a pale blue precipitate. Excess ammonia dissolves it to a deep blue solution.
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