AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Transition metal ions can be identified by their colour.

    Full specification wording

    Formation of coloured ions (A-level only): Transition metal ions can be identified by their colour.

    Each combination of metal, oxidation state, ligand and co-ordination number gives a different d orbital splitting and therefore a different colour, making colour a genuine identification tool.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 1 of 6
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Each combination of metal, oxidation state, ligand and co-ordination number gives a different d orbital splitting and therefore a different colour, making colour a genuine identification tool. Learn the aqueous ions first: [Cu(H₂O)₆]²⁺ is blue, [Fe(H₂O)₆]²⁺ pale green, [Co(H₂O)₆]²⁺ pink, [Cr(H₂O)₆]³⁺ ruby/green and [Mn(H₂O)₆]²⁺ very pale pink. For [Fe(H₂O)₆]³⁺, the pure ion is purple, but it often appears yellow or orange-brown in solution due to hydrolysis; AQA accepts these colours. Then learn the changes: excess ammonia turns copper(II) deep blue as [Cu(NH₃)₄(H₂O)₂]²⁺ forms, concentrated hydrochloric acid turns it yellow as [CuCl₄]²⁻ forms, and cobalt(II) goes from pink to blue. Oxidation state matters too, since MnO₄⁻ is purple while Mn²⁺ is almost colourless.

    What earns the marks

    • The correct colour of a named aqueous transition metal ion.
    • The colour of the product complex after a named reagent has been added.
    • Identifying the ion present from a described colour and a stated colour change.
    • Naming the change responsible, such as a change of ligand or of oxidation state.

    Common mistakes and how to put them right

    • Describing a solution colour as a precipitate colour, or the other way round.

    • Writing clear instead of colourless, which examiners will not accept as a colour.

    • Assuming every transition metal ion is intensely coloured, when Mn²⁺ is very pale and Zn²⁺ is not a transition metal ion at all.

    • Confusing the colours of iron(II) and iron(III) aqueous ions, or thinking orange-brown is incorrect for iron(III).

    Exam tips

    • State the full formula of the complex alongside the colour, because a colour on its own often scores only half.
    • Use colourless for a solution with no colour and reserve white for a solid.
    • Learn colours in pairs, starting solution and finishing solution, because questions almost always ask for a change.

    Try the lesson

    The start of the Formation of coloured ions (A-level only): Transition metal ions can be identified by their colour. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A student has three unlabelled aqueous solutions, each containing a different transition metal ion. One solution is blue, one is pale green and one is pale pink. Which statement best explains why the student can use these colours to help identify the ions present?

    Listen

    Each combination of metal, oxidation state, ligand and co-ordination number gives a different d orbital splitting and therefore a different colour, making colour a genuine identification tool. Learn the aqueous ions first: [Cu(H₂O)₆]²⁺ is blue, [Fe(H₂O)₆]²⁺ pale green, [Co(H₂O)₆]²⁺ pink, [Cr(H₂O)₆]³⁺ ruby/green and [Mn(H₂O)₆]²⁺ very pale pink. For [Fe(H₂O)₆]³⁺, the pure ion is purple, but it often appears yellow or orange-brown in solution due to hydrolysis; AQA accepts these colours. Then learn the changes: excess ammonia turns copper(II) deep blue as [Cu(NH₃)₄(H₂O)₂]²⁺ forms, concentrated hydrochloric acid turns it yellow as [CuCl₄]²⁻ forms, and cobalt(II) goes from pink to blue. Oxidation state matters too, since MnO₄⁻ is purple while Mn²⁺ is almost colourless. A strong answer earns marks for these points. the correct colour of a named aqueous transition metal ion. the colour of the product complex after a named reagent has been added. identifying the ion present from a described colour and a stated colour change. naming the change responsible, such as a change of ligand or of oxidation state. Exam tip: State the full formula of the complex alongside the colour, because a colour on its own often scores only half.

    See it

    A flowchart with two separate chains. In the first, the blue ion [Cu(H₂O)₆]²⁺ branches two ways: adding excess ammonia gives the deep blue complex [Cu(NH₃)₄(H₂O)₂]²⁺, and adding concentrated hydrochloric acid gives the yellow complex [CuCl₄]²⁻. In the second chain, pink [Co(H₂O)₆]²⁺ changes to blue cobalt(II). A third chain shows MnO₄⁻ as purple changing to Mn²⁺, which is almost colourless.
    Shows the colour changes of copper(II) and cobalt(II) complexes on adding named reagents, and the colour difference between two manganese oxidation states.

    Key words

    Say what each word means before you turn the card.

    identification by colour

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    aqueous complexes

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    colour changes on adding reagents

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    oxidation state and colour

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    See every definition
    identification by colour
    Transition metal ions can be identified by their colour.
    aqueous complexes
    The aqueous ions include [Cu(H₂O)₆]²⁺, which is blue, and [Fe(H₂O)₆]²⁺, which is pale green.
    colour changes on adding reagents
    Excess ammonia turns copper(II) deep blue, concentrated hydrochloric acid turns it yellow, and cobalt(II) goes from pink to blue.
    oxidation state and colour
    Oxidation state matters too, since MnO₄⁻ is purple while Mn²⁺ is almost colourless.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart with two separate chains. In the first, the blue ion [Cu(H₂O)₆]²⁺ branches two ways: adding excess ammonia gives the deep blue complex [Cu(NH₃)₄(H₂O)₂]²⁺, and adding concentrated hydrochloric acid gives the yellow complex [CuCl₄]²⁻. In the second chain, pink [Co(H₂O)₆]²⁺ changes to blue cobalt(II). A third chain shows MnO₄⁻ as purple changing to Mn²⁺, which is almost colourless.

    • 1Box 1

    • 2Box 2

    • 3Box 3

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