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Testing for Aqueous Cations

Chemistry: Edexcel IGCSE, topic 13.2.5

Testing for Aqueous Cations

Aqueous solutions of metal compounds contain metal ions. We can test what these metal ions are when they react with a sodium hydroxide solution (NaOH) to produce different coloured precipitates.

Tests for cations

Tests for cations

  • Copper (II) ions (Cu2+) form a blue precipitate when reacted with NaOH.
  • Iron (II) ions (Fe2+) form a green precipitate when reacted with NaOH.
  • Iron(III) ions (Fe3+) form a brown precipitate when reacted with NaOH.
Testing for ammonium ions

Testing for ammonium ions

  • Ammonium (NH4+) ions form water and ammonia when reacted with NaOH. Ammonia gas evolves out of the solutions.
  • You then test for ammonia gas which will turn damp red litmus paper from red to blue.

Test yourself on Testing for Aqueous Cations

  • I test an unknown solution with a few drops of sodium hydroxide. A blue precipitate forms. What ions are present in the unknown solution?

  • I test an unknown solution with a few drops of sodium hydroxide. A brown precipitate forms. What ions are present in the unknown solution?

  • How do you test a gas to check if it's ammonia?

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