2.6.3Inorganic Chemistry
Identifying Transition Metal Ions — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 27 of 27
Part 1 of 5Systematic Approach
1
Record the colour of the initial solution, because aqua ions have characteristic colours.
2
Add sodium hydroxide dropwise to a sample, note the precipitate colour, then add excess and record any change.
3
Repeat with ammonia solution, noting whether the precipitate dissolves and the colour of any solution.
4
Add sodium carbonate solution to see whether a carbonate forms or whether gas is evolved.
5
Compare all observations with known results to identify the ion.
Part 2 of 5Decision Table
Pale blue precipitate
- Cu²⁺; the precipitate dissolves in excess ammonia to give a deep blue solution.
Green precipitate turning brown
- Fe²⁺; the surface oxidises to Fe³⁺ on standing in air.
Brown precipitate
- Fe³⁺; the precipitate is insoluble in excess of both reagents.
White precipitate soluble in excess NaOH
- Al³⁺; it remains as a precipitate in excess ammonia.
Green precipitate soluble in excess NaOH
- Cr³⁺; a green solution is formed.
| Ion | NaOH | NH₃(aq) excess |
|---|---|---|
| Cu²⁺ | Pale blue ppt | Deep blue solution |
| Fe²⁺ | Green ppt | Green ppt |
| Fe³⁺ | Brown ppt | Brown ppt |
| Al³⁺ | White ppt dissolves | White ppt remains |
| Co²⁺ | Blue-green ppt | Straw solution, browns |
Part 3 of 5Carbonate and Anion Tests
Carbonate and Anion Tests
2+ ions
- Cu²⁺ and Fe²⁺ give carbonate precipitates with sodium carbonate solution, with no gas.
3+ ions
- Fe³⁺ and Al³⁺ give hydroxide precipitates and fizzing from carbon dioxide.
Counter ions
- Barium chloride tests for sulfate and silver nitrate for halide, both after acidifying with a suitable acid.
Part 4 of 5Evaluating Results
Evaluating Results
Careful wording
- Colour and appearance must be precise, such as pale blue precipitate rather than blue solid.
Oxidation
- Standing in air can change an Fe²⁺ precipitate, so record observations promptly and again after waiting.
Confirmation
- Use at least two independent tests, as several ions give green or brown precipitates.
Part 5 of 5Exam Checklist & Tips
2.6.3 – Key Points for the Exam
- Unknown solution A sample whose ions are to be identified from observations.
- Ammonia solution A weak base and ligand used to form precipitates and ammine complexes.
- Amphoteric Able to react as an acid and as a base.
- Observation What is seen, such as a precipitate colour, not an explanation.
Common Mistakes
- Reporting inferences like Cu²⁺ present as observations.
- Adding excess reagent at once and missing the initial precipitate.
- Confusing Fe²⁺ (green) and Fe³⁺ (brown) precipitates.
- Assuming that a precipitate dissolving in ammonia means aluminium is present.
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