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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.

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Key term

What is qualitative analysis?

Key term

Where is qualitative analysis used in industry and forensic science? Give three examples.

Key term

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. 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. 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. 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. 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. 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. 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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