GCSE Chemistry (AQA)

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Metal Hydroxide Test

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Chemical analysis Identification of ions by chemical and spectroscopic means

Metal Hydroxide Test

7:26 Tests for Ions
Spec 4.8.3.2
Previous Flame Tests Next Testing for Anions
  • Sodium hydroxide solution can identify positive metal ions.
  • This test is important for exams.
  • Sodium hydroxide (NaOH) is added drop by drop to an unknown solution.
  • A precipitate forms if certain metal ions are present.
  • Aluminum, calcium, and magnesium ions form white precipitates with NaOH.
  • Calcium ions form calcium hydroxide, a white precipitate.
  • Balanced equation for calcium: CaSO4 + 2NaOH ��� Ca(OH)2 + Na2SO4.
  • Magnesium ions also form a white precipitate.
  • Balanced equation for magnesium: MgSO4 + 2NaOH ��� Mg(OH)2 + Na2SO4.
  • Aluminum ions form a white precipitate that dissolves with excess NaOH.
  • Balanced equation for aluminum: Al2(SO4)3 + 6NaOH ��� 2Al(OH)3 + 3Na2SO4.
  • Calcium and magnesium hydroxide precipitates do not dissolve with excess NaOH.
  • Copper(II) ions form a blue precipitate with NaOH.
  • Balanced equation for copper: CuSO4 + 2NaOH ��� Cu(OH)2 + Na2SO4.
  • Iron(II) ions form a green precipitate with NaOH.
  • Balanced equation for iron(II): FeSO4 + 2NaOH ��� Fe(OH)2 + Na2SO4.
  • Iron(III) ions form a brown precipitate with NaOH.
  • Balanced equation for iron(III): Fe2(SO4)3 + 6NaOH ��� 2Fe(OH)3 + 3Na2SO4.
  • Aluminum hydroxide dissolves with excess NaOH, distinguishing it from calcium and magnesium.

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