Qualitative analysis tests for ions: flame tests for metal cations, tests for cations using sodium hydroxide, tests for anions (carbonate, sulfate, halide), and identifying ions in an unknown salt.

What this dot point is asking

Edexcel wants you (in separate chemistry) to carry out flame tests for metal ions, use sodium hydroxide solution to test for metal cations and recall the precipitate colours, test for the carbonate, sulfate and halide anions, write ionic equations for the precipitates, and identify the ions in an unknown salt. The flame colours and precipitate tests are the central marks.

Flame tests for metal ions

A clean wire is dipped in the sample and held in a blue Bunsen flame; the colour identifies the metal:

The wire must be cleaned (in acid) between tests so colours are not mixed, and only one ion can be tested at a time because a strong colour can mask a weaker one.

Sodium hydroxide test for cations

Adding sodium hydroxide solution to a solution of the metal ion forms a metal hydroxide precipitate whose colour identifies the metal:

• Copper(II) ($Cu^{2+}$): blue precipitate, $Cu^{2+} + 2OH^- \rightarrow Cu(OH)_2$.
• Iron(II) ($Fe^{2+}$): green precipitate, $Fe^{2+} + 2OH^- \rightarrow Fe(OH)_2$.
• Iron(III) ($Fe^{3+}$): brown precipitate, $Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3$.
• Calcium ($Ca^{2+}$) and aluminium ($Al^{3+}$): white precipitates; the aluminium hydroxide redissolves in excess sodium hydroxide, which distinguishes it from calcium.

Tests for anions

Three anion tests you must know:

• Carbonate ($CO_3^{2-}$): add dilute acid; it fizzes, releasing carbon dioxide, which turns limewater cloudy.
• Sulfate ($SO_4^{2-}$): add dilute hydrochloric acid then barium chloride solution; a white precipitate of barium sulfate confirms sulfate.
• Halides ($Cl^-$, $Br^-$, $I^-$): add dilute nitric acid then silver nitrate solution; the precipitate colour identifies the halide:

Identifying an unknown salt

To identify an unknown ionic compound, test for the cation (flame test or sodium hydroxide) and the anion (carbonate, sulfate or halide test) separately, then combine the results. For example, a lilac flame and a white precipitate with acidified barium chloride means potassium sulfate.

Try this

Q1. State the flame colour for potassium and for lithium. [2 marks]

• Cue. Potassium lilac; lithium red.

Q2. Describe the test for sulfate ions and the positive result. [2 marks]

• Cue. Add dilute hydrochloric acid then barium chloride solution; a white precipitate confirms sulfate.

Q3. A metal ion gives a brown precipitate with sodium hydroxide. Identify the ion and write the ionic equation. [2 marks]

• Cue. Iron(III), $Fe^{3+}$; $Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3$.