GCSE Chemistry · AQA 8462 · Topic 4.8.3
Identification of ions by chemical and spectroscopic means
Key points
- Flame tests identify metal ions: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green. A mixture of ions may mask some colours.
- Sodium hydroxide solution forms coloured precipitates (hydroxides): copper(II) blue, iron(II) green, iron(III) brown. Aluminium, calcium and magnesium give white precipitates; only aluminium hydroxide dissolves in excess NaOH.
- Carbonates: add dilute acid; the gas produced turns limewater milky.
- Halides: add dilute nitric acid then silver nitrate. Chloride gives a white, bromide a cream and iodide a yellow precipitate.
- Sulfates: add dilute hydrochloric acid then barium chloride solution; a white precipitate forms.
- Instrumental methods are accurate, sensitive and rapid. Flame emission spectroscopy passes light from a flame through a spectroscope; the line spectrum identifies ions and can measure concentration.
Topic notesPremium
Many compounds are white solids that look identical, so chemists use tests to identify the ions in them. You need the flame test colours, the sodium hydroxide precipitates, the tests for carbonate, halide and sulfate ions, and the idea…
Unlock the full notes with Premium
Premium includes full topic notes, the paper generator, self-quizzes and progress tracking.
Practise this topicPremium
Premium members can generate an exam-style paper with a mark scheme, or take a quick quiz, on Identification of ions by chemical and spectroscopic means.