Part 1: Plan and interpret a complete ion-testing sequence

Combine Year 12 anion tests with transition-metal and ammonium tests. Use separate aliquots and record evidence that actually distinguishes the candidates.

A useful test must answer a specific question

Qualitative analysis identifies species from reproducible reactions. Work on small labelled portions of the unknown, record starting appearance, add reagent a little at a time and then in excess when that distinction matters. A fresh aliquot prevents reagents from one test creating false positives in another.

Distinguish observation from inference. “A pale-blue precipitate forms and dissolves in excess ammonia to a deep-blue solution” is an observation sequence; “Cu²⁺ is present” is its interpretation. Write both when asked. One observation may fit more than one ion, so combine independent evidence.

This topic develops PAG4 skills. Wear the required eye protection, use the specified small quantities and follow laboratory risk assessments for corrosive reagents, ammonia and metal-ion waste. Heating a sample requires appropriate handling; do not identify gases by directly inhaling them.

Choose an acid that does not supply the ion being tested

For carbonate, add dilute acid and pass the evolved gas through limewater; effervescence plus limewater turning milky supports CO₂. For sulfate, use an acidified fresh aliquot and barium ions; a white BaSO₄ precipitate supports sulfate. Acidification removes carbonate that could otherwise form white BaCO₃.

For halides, acidify a fresh portion with dilute nitric acid and add silver nitrate. HCl would introduce chloride and could create a false AgCl result. Use ammonia to distinguish the precipitates rather than relying on subtle colour differences alone. Do not use sulfuric acid to prepare a sulfate-test aliquot.

Release ammonia from ammonium with an alkali

Add aqueous NaOH to a separate portion and warm gently as directed. Ammonium ions react to release ammonia, which turns damp red litmus blue. The paper must be damp for the gas to dissolve and establish alkaline conditions; avoid touching it with alkaline solution splashes.

NH₄⁺ + OH⁻ → NH₃ + H₂O shows proton transfer. Ammonium is the conjugate acid and hydroxide removes a proton. Do not confuse this gas test with adding aqueous ammonia as a reagent to precipitate transition-metal hydroxides.

Use both the initial precipitate and excess-reagent behaviour

The same initial hydroxides form with small amounts of NaOH or ammonia. The excess tests distinguish copper and chromium from iron and manganese. Look promptly because Fe(OH)₂ and Mn(OH)₂ can change colour through air oxidation. Chromium–ammonia ligand replacement may develop slowly under the specified conditions.

A green initial precipitate alone is not enough to distinguish Fe²⁺ from Cr³⁺. Solubility in excess NaOH supports chromium; iron(II) hydroxide remains. A blue precipitate becoming a deep-blue solution in excess ammonia strongly supports copper(II) among this required set.

Worked interpretation: combine cation and anion evidence

An original unknown gives a grey-green precipitate with NaOH that dissolves in excess to a green solution. A separate acidified sample gives a white precipitate with barium chloride. The cation evidence supports Cr³⁺ and the anion evidence supports SO₄²⁻. For a simple salt containing only those ions, charge balance gives Cr₂(SO₄)₃.

Explain why the alternatives fail: Fe²⁺ can give green hydroxide but it does not dissolve in excess NaOH in this scheme. A white barium precipitate without prior acidification would be less decisive because carbonate is an alternative. These tests do not establish a hydration number or prove the absence of every trace impurity.

For a second illustrative sample, white AgCl dissolving in dilute ammonia and copper’s characteristic ammonia sequence support CuCl₂, assuming a simple salt. Use separate portions; putting HCl into the halide aliquot would destroy the value of the chloride inference.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources