Tests for cations and anions including carbonate, sulfate, halide, ammonium and hydroxide, tests for common gases, flame tests, and the chemistry behind each observation.

What this dot point is asking

CCEA wants you to carry out and interpret tests for common cations and anions, including carbonate, sulfate, halide, ammonium and hydroxide ions, tests for common gases, and flame tests, explaining the observations and the chemistry behind each.

Tests for anions

The nitric acid removes carbonate or hydroxide ions that would otherwise give a false precipitate. The chemistry behind each test is worth knowing as an ionic equation: the carbonate test is $\text{CO}_3^{2-} + 2\text{H}^+ \rightarrow \text{H}_2\text{O} + \text{CO}_2$; the sulfate test is $\text{Ba}^{2+} + \text{SO}_4^{2-} \rightarrow \text{BaSO}_4$ (white); and the halide test is $\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl}$ (and similarly for bromide and iodide). The silver halides are then distinguished by their solubility in ammonia: silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide stays insoluble even in concentrated ammonia. This ammonia step mirrors the trend that the silver halides become less soluble down Group VII.

Tests for cations

The ammonium test works because the hydroxide ion is a strong enough base to displace ammonia from the ammonium ion: $\text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O}$. The ammonia is detected by its smell and by turning damp red litmus blue. The metal hydroxide precipitates form because most metal hydroxides are insoluble: iron(II) gives a green precipitate that slowly turns brown as it is oxidised in air, iron(III) gives a red-brown precipitate, and copper(II) gives a pale blue precipitate. The colour of the precipitate, together with whether it redissolves in excess sodium hydroxide, narrows down the cation.

Tests for gases

• Oxygen: relights a glowing splint.
• Hydrogen: burns with a squeaky pop using a lighted splint.
• Carbon dioxide: turns limewater milky.
• Ammonia: turns damp red litmus blue and has a pungent smell.
• Chlorine: bleaches damp blue litmus paper.

Flame tests

Examples in context

Example 1. Firework colours. The colours in a firework are flame tests on a large scale: strontium and lithium salts give red, sodium salts give yellow, barium salts give green, and copper salts give blue-green. The metal ions are excited by the heat of the burning fuel and emit light of characteristic wavelengths as their electrons fall back. CCEA expects candidates to explain firework colours with the same electron-excitation reasoning used for the laboratory flame test.

Example 2. Detecting hard water and sulfate pollution. Water companies use the acidified barium chloride test to measure sulfate levels in drinking water and in industrial discharge, because barium sulfate's insolubility makes the white precipitate a sensitive indicator. The same insoluble barium sulfate, formed in the laboratory test, links directly to the Group II trend that sulfate solubility falls down the group. This shows how a simple qualitative test underpins real environmental monitoring.

Try this

Q1. Describe the test for a sulfate ion and the positive result. [2 marks]

• Cue. Add acidified barium chloride; a white precipitate of barium sulfate forms.

Q2. State the flame colour for potassium. [1 mark]

• Cue. Lilac.