Tests for gases and ions

Chemists identify substances with simple tests. Each common gas has its own test. Metal ions can be found with flame tests and by adding sodium hydroxide solution to see the colour of the precipitate. Negative ions are found by adding acids and other solutions: carbonates fizz with acid, halides give coloured precipitates with silver nitrate, and sulfates give a white precipitate with barium chloride.

Key facts

  • Gases: hydrogen burns with a squeaky pop; oxygen relights a glowing splint; carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper.
  • Flame tests: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green.
  • With sodium hydroxide solution: copper(II) blue, iron(II) green, iron(III) brown precipitates; aluminium, calcium and magnesium white. Only aluminium hydroxide dissolves in excess.
  • Carbonates: add dilute acid; the gas (carbon dioxide) turns limewater milky.
  • Halides: add dilute nitric acid then silver nitrate: chloride white, bromide cream, iodide yellow precipitate.
  • Sulfates: add dilute hydrochloric acid then barium chloride: a white precipitate.
  • Flame emission spectroscopy is an instrumental method: it identifies metal ions in mixtures and measures their concentrations.

Worked examples

Example 1

A white solid gives a lilac flame. Its solution gives a cream precipitate with dilute nitric acid and silver nitrate. Name the solid.

  1. A lilac flame shows potassium ions, K⁺.
  2. A cream precipitate with silver nitrate shows bromide ions, Br⁻ (silver bromide). Ag⁺ + Br⁻ → AgBr
  3. So the solid is potassium bromide, KBr.

Answer: potassium bromide

Example 2

Sodium hydroxide solution is added to solutions P, Q and R. P gives a blue precipitate, Q a brown one and R a white one that dissolves in excess. Which metal ions are present?

  1. Blue: copper(II) hydroxide, so P contains Cu²⁺. Cu²⁺ + 2OH⁻ → Cu(OH)2
  2. Brown: iron(III) hydroxide, so Q contains Fe³⁺. Fe³⁺ + 3OH⁻ → Fe(OH)3
  3. White, dissolving in excess: aluminium hydroxide, so R contains Al³⁺.

Answer: P copper(II), Q iron(III), R aluminium

Example 3

Why is hydrochloric acid added before barium chloride when testing for sulfate ions?

  1. Carbonate ions also form a white precipitate with barium ions (barium carbonate).
  2. The acid reacts with any carbonate and removes it as carbon dioxide.
  3. Hydrochloric acid is used, not sulfuric acid, because sulfuric acid contains sulfate ions itself.

Answer: to remove carbonate ions, which would also give a white precipitate