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Learn5070Paper 1 · Multiple ChoiceExperimental techniques and chemical analysisTests for ions and gases
Experimental techniques and chemical analysis · Subtopic 3 of 3

Tests for Ions and Gases

Flame tests: Li red, Na yellow, K lilac, Ca orange-red, Ba light green, Cu blue-green. Sodium hydroxide: Cu2+\mathrm{Cu^{2+}} blue ppt, Fe2+\mathrm{Fe^{2+}} green ppt, Fe3+\mathrm{Fe^{3+}} red-brown ppt, Al3+\mathrm{Al^{3+}}/Zn2+\mathrm{Zn^{2+}} white ppt (dissolves in excess), NH4+\mathrm{NH_4^+} ammonia on warming. Anions: carbonate + acid → CO2\mathrm{CO_2}; halides + acidified silver nitrate → white (Cl⁻), cream (Br⁻), yellow (I⁻); sulfate + acidified barium chloride/nitrate → white ppt. Gases: H₂ squeaky pop; O₂ relights glowing splint; CO₂ limewater milky; Cl₂ bleaches damp litmus; NH₃ turns damp red litmus blue.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Identify cations with flame tests and hydroxide precipitates.
  • Identify anions with acid, silver nitrate and barium tests.
  • Identify common gases.
Pause and think

Why do fireworks come in so many colours?

Section 2 of 14

Core idea

Start hereChemical detective work

A forensic chemist is handed an unlabelled white powder. With a wire loop, a few drops of reagent and a test tube, she can name it in minutes.

The big idea

Each ion and gas gives a distinctive result with a specific test. Combine a cation test and an anion test to identify a salt.

1. Cations

  • Flame: Li red, Na yellow, K lilac, Ca orange-red, Ba light green, Cu blue-green.
  • NaOH: Cu²⁺ light blue; Fe²⁺ green; Fe³⁺ red-brown; Ca²⁺ white (insoluble in excess); Al³⁺/Zn²⁺ white (soluble in excess).
  • NH₄⁺: warm with NaOH → ammonia (damp red litmus → blue).

2. Anions

Anion test sequence

Acidify first to remove carbonate.

Worked exampleName the compound

A solution gives a green precipitate with NaOH and a white precipitate with acidified barium chloride.

Goal
Identify it.
Strategy
Combine the results.
  1. green ppt⇒Fe2+\text{green ppt}\Rightarrow\mathrm{Fe^{2+}}
    Iron(II) hydroxide.
  2. white with Ba2+⇒SO42−\text{white with }\mathrm{Ba^{2+}}\Rightarrow\mathrm{SO_4^{2-}}
    Barium sulfate.
AnswerIron(II) sulfate, FeSO4\mathrm{FeSO_4}.
✓ CheckGreen precipitate darkens in air as it oxidises to iron(III) ✓.

3. Gases

  • H₂: lit splint, squeaky pop.
  • O₂: relights glowing splint.
  • CO₂: limewater milky.
  • Cl₂: bleaches damp litmus.
  • NH₃: damp red litmus → blue.
  • SO₂: acidified KMnO4\mathrm{KMnO_4} purple → colourless.
Common misconceptionLit vs glowing Hydrogen: lit splint. Oxygen: glowing splint. Mixing these up is the most common error.
Pause and think

Why does sodium contamination make flame tests difficult?

Summary

  • Flame tests and NaOH for cations.
  • Acid, AgNO₃, BaCl₂ for anions.
  • Six gas tests.
Key words
Precipitate
Insoluble solid formed in a solution.
Flame test
Identifying metal ions by flame colour.
Acidified
Dilute acid added first.
HistoryBunsen and Kirchhoff. Using flame colours through a prism, Robert Bunsen and Gustav Kirchhoff discovered caesium (1860) and rubidium (1861), named from Latin for "sky blue" and "deep red".
Fun factHelium in the Sun. Helium was found in the Sun's spectrum in 1868, 27 years before it was found on Earth. Its name comes from Helios, the Greek Sun god.
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