GCSE Chemistry · Edexcel 1CH0 · Topic 9A
Tests for ions
Key points
- Flame tests: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper blue-green.
- Sodium hydroxide solution gives coloured precipitates: copper(II) blue, iron(II) green, iron(III) orange-brown; aluminium and calcium white (aluminium's dissolves in excess).
- Ammonium ions: warm with sodium hydroxide; ammonia gas turns damp red litmus blue.
- Carbonate: add dilute acid; CO₂ turns limewater milky. Sulfate: add dilute HCl then barium chloride: white precipitate.
- Halides: add dilute nitric acid then silver nitrate: chloride white, bromide cream, iodide yellow.
- Flame photometry is an instrumental method that measures concentration from flame emission intensity; it is quicker and more sensitive than a flame test and works on mixtures.
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Chemists identify an unknown ionic compound by testing for its positive ion (cation) and its negative ion (anion) separately. These notes give the tests you must know, what a positive result looks like, and how instrumental methods compare…
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