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4.3.3Electrochemistry

Sodium Hydroxide Test — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 5 of 20 Unit quiz

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

4.3.3 Sodium Hydroxide Test

Adding the reagent in two stages reveals whether a precipitate dissolves in excess.

1
Place a small sample of the metal ion solution in a test tube and add a few drops of sodium hydroxide solution.
2
Record the colour and appearance of any precipitate that forms, noting whether it is soluble or insoluble.
3
Add sodium hydroxide solution in excess and shake the tube, then record whether the precipitate has dissolved.
4
Compare the observations with a table of known results to identify the metal ion present.

Part 2 of 5Precipitate Colours and Solubility

The colour of the hydroxide precipitate and its solubility in excess are both used to identify the ion.

Copper(II), Cu²⁺
  • Light blue precipitate that does not dissolve in excess sodium hydroxide.
Iron(II), Fe²⁺
  • Green precipitate that does not dissolve in excess sodium hydroxide.
Iron(III), Fe³⁺
  • Brown precipitate that does not dissolve in excess sodium hydroxide.
Calcium, Ca²⁺
  • White precipitate that remains in excess sodium hydroxide.
Magnesium, Mg²⁺
  • White precipitate that remains in excess sodium hydroxide, so it looks the same as calcium at this stage.
Aluminium, Al³⁺
  • White precipitate that dissolves in excess sodium hydroxide to form a colourless solution.

Part 3 of 5Telling Similar Results Apart

Telling Similar Results Apart
Aluminium
  • The only common ion here whose precipitate dissolves in excess, so it is the easiest to recognise.
Calcium and magnesium
  • Both give white precipitates that do not dissolve, so a second test is needed to tell them apart.
Flame test
  • A flame test can separate calcium, which gives a red flame, from magnesium, which gives no characteristic flame colour.
Slow addition
  • Adding sodium hydroxide slowly keeps the first precipitate visible before it might dissolve in excess.

Part 4 of 5Worked Example

Question

  • A solution gives a white precipitate with a few drops of sodium hydroxide.
  • The precipitate dissolves completely when more sodium hydroxide is added.
  • Which metal ion could be present, and what second test could confirm it?

  1. A white precipitate that dissolves in excess sodium hydroxide is the result for aluminium, Al³⁺.
  2. Calcium and magnesium would stay as white precipitates, so they are ruled out.
  3. Write the first step as Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s), which then dissolves in excess.

Answer Aluminium, Al³⁺, is the most likely ion, because its white precipitate dissolves in excess NaOH.

Part 5 of 5Exam Checklist & Tips

4.3.3 – Key Points for the Exam

  • Hydroxide precipitate An insoluble metal hydroxide that forms when sodium hydroxide reacts with a metal ion solution.
  • Excess More of a reagent than is needed to react with all of the sample.
  • Soluble Able to dissolve in a solvent, such as water, to form a solution.
  • Colourless
    • Having no colour at all;
      • a solution can be clear, meaning see-through, and still be coloured.

Common Mistakes

  • Adding sodium hydroxide in one large portion, which can dissolve a precipitate before it is noticed.
  • Stating that a solution is colourless when it is actually clear, or the other way round.
  • Assuming calcium and magnesium can be told apart by the sodium hydroxide test alone.
  • Confusing the colour of the solution with the colour of the precipitate.

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