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Transition Metals

Hydroxide precipitates with sodium hydroxide

  • 3 things to know
  • Unit A2 2

Overview

Seen when sodium hydroxide is added to chromium(III) solution, seen when sodium hydroxide is added to manganese(II) solution and seen when sodium hydroxide is added to iron(II) solution.

What you need to know

Answer each one before you reveal it.

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Key term

What happens in general when sodium hydroxide solution is added to a transition metal salt?

Key term

Write the general ionic equation for the precipitation of a two plus metal ion by sodium hydroxide

Key term

What is seen when sodium hydroxide is added to chromium(III) solution?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What happens in general when sodium hydroxide solution is added to a transition metal salt?

    Model answer

    The metal hydroxide is precipitated, and its colour helps to identify the metal.

  2. 2

    Write the general ionic equation for that precipitation for a two plus ion.

    Model answer

    M²⁺(aq) + 2OH⁻(aq) → M(OH)₂(s).

  3. 3

    What is seen when sodium hydroxide is added to chromium(III) solution?

    Model answer

    A green precipitate of chromium(III) hydroxide forms.

  4. 4

    What happens when excess sodium hydroxide is added to that precipitate?

    Model answer

    It dissolves to give a green solution, because chromium(III) hydroxide is amphoteric.

  5. 5

    Why does chromium(III) hydroxide dissolve in excess alkali?

    Model answer

    It is amphoteric, so it reacts with the hydroxide ions to form a soluble chromate(III) complex.

  6. 6

    What is seen when sodium hydroxide is added to manganese(II) solution?

    Model answer

    An off-white precipitate forms which slowly darkens to brown in air.

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