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2.8.3Atoms, Elements and Compounds

Tests for Cations — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 20 of 26 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Sodium Hydroxide Test

Adding a few drops of sodium hydroxide forms a precipitate with many metal ions, and the colour identifies the ion.

Iron(II)
  • Iron(II) Fe²⁺ gives a pale green precipitate of iron(II) hydroxide, Fe(OH)₂, which is insoluble.
Iron(III)
  • Iron(III) Fe³⁺ gives an orange-brown precipitate of iron(III) hydroxide, Fe(OH)₃, which is insoluble.
Copper(II)
  • Copper(II) Cu²⁺ gives a light blue precipitate of copper(II) hydroxide, Cu(OH)₂, which is insoluble.
No precipitate
  • No precipitate If no precipitate forms, the metal hydroxide may be soluble or the ion may be absent.
IonObservation with NaOHIonic equation
Fe²⁺Pale green precipitateFe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)
Fe³⁺Orange-brown precipitateFe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)
Cu²⁺Light blue precipitateCu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

Part 2 of 5Testing for Ammonium

Ammonium compounds release ammonia gas when warmed with sodium hydroxide.

1
Add sodium hydroxide solution to the sample in a test tube and warm the mixture gently.
2
Ammonia gas is released if ammonium ions are present, with a choking smell.
3
Hold damp red litmus paper near the mouth of the tube, without touching the sides, and check for a colour change.
4
Damp red litmus turns blue, which confirms that the gas is ammonia and so the sample contains ammonium ions.

Part 3 of 5Equations and Caution

Equations and Caution
Ammonium ion
  • Ammonium ion NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l), which is only seen when the mixture is warmed.
Precipitate colour
  • Precipitate colour The colour of the copper(II) precipitate must be stated as light blue to gain full credit in examinations.
Small amounts
  • Small amounts A small amount of ion may give only a faint cloudiness, which can still be a positive result.
Hazard
  • Hazard Sodium hydroxide is corrosive, so eye protection must be worn when carrying out the test.

Part 4 of 5Worked Example

Question

  • An unknown solution forms a pale green precipitate with sodium hydroxide solution, which darkens on standing.
  • Identify the cation and write the ionic equation for its reaction.

  1. A pale green precipitate with sodium hydroxide is the test for iron(II) ions.
  2. The precipitate darkens as the iron(II) hydroxide is oxidised by air, which is a secondary observation.
  3. Ionic equationFe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s).

Answer

  • Iron(II) ions, Fe²⁺.
  • Ionic equation: Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s).

Part 5 of 5Exam Checklist & Tips

2.8.3 – Key Points for the Exam

  • Precipitate An insoluble solid formed when two solutions react together.
  • Cation A positively charged ion.
  • Ammonium ion The NH₄⁺ ion, which releases ammonia gas when warmed with an alkali.
  • Hydroxide precipitate An insoluble metal hydroxide formed when sodium hydroxide reacts with a metal ion.

Common Mistakes

  • Describing the iron(II) precipitate as dark brown; the iron(II) precipitate is pale green.
  • Confusing the iron(II) and iron(III) precipitates, which have different colours.
  • Testing for ammonium without warming, so no ammonia gas is released.
  • Touching the sides of the test tube with litmus paper, which may give a false positive from sodium hydroxide.

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