Solutions of copper(II), iron(II) and iron(III) ions form coloured…

The idea

Transition metal ions in aqueous solution can be identified by adding a few drops of dilute sodium hydroxide solution. The hydroxide ions react with the metal aqua ions to form an insoluble metal hydroxide, which appears as a coloured precipitate because the solid particles are dispersed through the liquid. Copper(II) ions give a blue precipitate, iron(II) ions give a green precipitate and iron(III) ions give a brown precipitate. For example, adding NaOH(aq) to copper(II) sulfate solution produces Cu(OH)₂(s), which settles as a blue solid. The colour is characteristic of the metal ion, so this test distinguishes the three ions from each other and from ions that give no precipitate or a white precipitate.

What earns the marks

• Sodium hydroxide solution supplies hydroxide ions that react with metal ions in solution.
• The product is an insoluble metal hydroxide, so it appears as a precipitate rather than remaining dissolved.
• The colour of the precipitate depends on the identity of the metal ion present.
• Copper(II) ions produce a blue precipitate, iron(II) ions produce a green precipitate and iron(III) ions produce a brown precipitate.
• The test can be used to distinguish between solutions containing different transition metal ions.

Common mistakes and how to put them right

• Describing the colour as belonging to the solution rather than the precipitate.

  Put it right: The colour change is caused by the insoluble solid formed, so the observation should refer to the precipitate.

• Confusing iron(II) and iron(III) colours.

  Put it right: Iron(II) hydroxide is green while iron(III) hydroxide is brown, so the oxidation state must be matched to the correct colour.

• Writing that sodium hydroxide dissolves in the metal solution without reaction.

  Put it right: The hydroxide ions react to form a new insoluble compound, so the word reaction or precipitate should be used.

Exam tips

• State the reagent as sodium hydroxide solution and the observation as a coloured precipitate, naming the colour precisely.
• Link each colour to the correct ion, including the oxidation state in the name, for example iron(II) for green and iron(III) for brown.
• If asked to identify an unknown, describe adding sodium hydroxide solution and observing the precipitate colour before naming the ion.