Tests for ions

Tests for ions identify what an ionic compound is made of. Metal ions (positive) are identified with flame tests or by adding sodium hydroxide solution and noting the colour of the precipitate. Negative ions are identified with three tests: dilute acid for carbonates, silver nitrate for halides, and barium chloride for sulfates. In AQA GCSE Chemistry these are part of required practical 7.

Flame tests

Dip a clean wire loop into the solid or solution and hold it in a hot, non-luminous Bunsen flame. AQA lists these colours:

Clean the loop between tests, for example by dipping it in hydrochloric acid and heating it until it gives no colour, so traces of one sample don't colour the next. AQA notes that if a sample contains a mixture of ions, some flame colours can be masked: a strong yellow from sodium can hide others.

Metal hydroxide precipitates

Add a few drops of sodium hydroxide solution to a solution of the compound. Many metal ions form an insoluble hydroxide, which appears as a coloured precipitate:

Three ions give white precipitates. Adding more sodium hydroxide tells aluminium apart, because only aluminium hydroxide dissolves in excess. To tell calcium from magnesium, use a flame test: calcium gives orange-red, magnesium no colour.

The precipitates form in reactions like these:

• Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) blue
• Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) brown

Tests for negative ions

Carbonate ions, CO₃²⁻

Add dilute acid, such as hydrochloric acid. Carbonates react to release carbon dioxide, so the mixture fizzes. Bubble the gas through limewater: carbon dioxide turns it milky (cloudy). The tests for gases guide covers the limewater test.

Halide ions: Cl⁻, Br⁻ and I⁻

Add dilute nitric acid, then silver nitrate solution. A precipitate means a halide is present, and its colour tells you which one:

The nitric acid removes any carbonate ions first, which would also make a precipitate with silver nitrate. Don't use hydrochloric acid here: it contains chloride ions and would give a white precipitate whatever the sample.

Sulfate ions, SO₄²⁻

Add dilute hydrochloric acid, then barium chloride solution. Sulfate ions give a white precipitate of barium sulfate: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The acid again removes carbonate ions first. Don't use sulfuric acid, because it contains sulfate ions.

Identifying an unknown compound

To identify a single ionic compound, test for the positive ion and the negative ion separately, then put the names together.

Example 1

Compound A gives a lilac flame. Its solution gives a cream precipitate with dilute nitric acid and silver nitrate. Lilac means potassium; cream means bromide. A is potassium bromide, KBr.

Example 2

Compound B's solution gives a green precipitate with sodium hydroxide, and a white precipitate with dilute hydrochloric acid and barium chloride. Green hydroxide means iron(II); the barium chloride result means sulfate. B is iron(II) sulfate, FeSO₄.

Example 3

Compound C gives a crimson flame. It fizzes with dilute acid, and the gas turns limewater milky. Crimson means lithium; the gas is carbon dioxide, so C contains carbonate ions. C is lithium carbonate, Li₂CO₃.

Instrumental methods: flame emission spectroscopy

Chemical tests work well for single compounds, but AQA notes that instrumental methods are accurate, sensitive and rapid. In flame emission spectroscopy, the sample is put into a flame and the light it gives out is passed through a spectroscope. The output is a line spectrum that can be analysed to identify the metal ions in the solution. Unlike a flame test, it can pick out several metals in a mixture.

Common mistakes

• Using hydrochloric acid in the halide test. It adds chloride ions. Use nitric acid.
• Using sulfuric acid in the sulfate test. It adds sulfate ions. Use hydrochloric acid.
• Forgetting the excess NaOH step for white precipitates: only aluminium hydroxide dissolves.
• Mixing up the iron ions. Iron(II) gives green; iron(III) gives brown.
• Calling copper's flame blue. Copper gives a green flame but a blue hydroxide precipitate.

To practise chemical analysis in a short timed paper, try today's GCSE Chemistry Higher or GCSE Chemistry Foundation paper.

Read next

Chemistry · AnalysisTest for hydrogen, oxygen, carbon dioxide and chlorineThe GCSE tests for gases: the squeaky pop test for hydrogen, a glowing splint for oxygen, limewater for carbon dioxide and damp litmus paper for chlorine.