Tests for gases and ions · Grade 11
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.