Transition elements - 5.3.2 Qualitative analysis — OCR A-Level Chemistry
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Transition elements - 5.3.2 Qualitative analysis explained
Transition elements - 5.3.2 Qualitative analysis covers 4 specification points, including Tests for ions and Processes and techniques needed to identify….
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Open a point below for its explanation and revision guidance.
Tests for ions
This row is a heading for the qualitative analysis work in 5.3.2, so treat it as a signpost to the ion tests the specification actually names rather than a list of every ion you have met. Read the numbered sub-statements under the heading and learn each test as a procedure: the reagent added, the observation, and the ionic equation where required. For example, you add dilute sodium hydroxide solution to a sample and record the colour of any precipitate, then test any gas released. Keep the cation and anion tests separate in your notes, and check the specification wording for which ions are required at this point.
(a) qualitative analysis of ions on a test-tube scale: processes and techniques needed to identify the following ions in an unknown compound:
This criterion asks you to carry out qualitative analysis of ions on a test-tube scale and to know the processes and techniques that let you identify ions in an unknown compound. In practice you dissolve a small sample, add a few drops of aqueous sodium hydroxide, record any precipitate colour, then add excess and note whether it dissolves; repeat with aqueous ammonia. For anions, acidify with dilute nitric acid and add silver nitrate to detect halides, or add barium chloride or barium nitrate with dilute acid to detect sulfate. You must also know the ionic equations, including formation of complex ions such as the tetraamminecopper(II) ion, and be able to explain why excess reagent is added and why acidification precedes the anion tests.
(i) anions: CO₃²⁻, Cl⁻, Br⁻, I⁻, SO₄²⁻ (see 3.1.4 a)
This row lists the anions whose qualitative tests you must know: carbonate (CO₃²⁻), chloride (Cl⁻), bromide (Br⁻), iodide (I⁻) and sulfate (SO₄²⁻). Learn each test by reagent, observation and ionic equation. Carbonate reacts with dilute acid to release carbon dioxide, which turns limewater milky. Chloride, bromide and iodide are identified by adding dilute nitric acid then silver nitrate: chloride gives a white precipitate, bromide a cream precipitate and iodide a pale yellow precipitate. Sulfate is identified by adding dilute hydrochloric acid then barium chloride solution, giving a white precipitate of barium sulfate. Write charges with superscript signs, for example CO₃²⁻ and SO₄²⁻, and balance each ionic equation.
(ii) cations: NH₄⁺; Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Cr³⁺ (see 3.1.4 a, 5.3.1 j).
This row lists the cations whose qualitative tests you must know: ammonium (NH₄⁺) and the transition metal ions Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺ and Cr³⁺. Ammonium is identified by warming with sodium hydroxide solution, releasing ammonia gas that turns damp red litmus paper blue. The metal ions are identified by adding sodium hydroxide solution and observing the hydroxide precipitate: Cu²⁺ gives a blue precipitate, Fe²⁺ a green precipitate, Fe³⁺ a reddish-brown precipitate, Mn²⁺ a pale pink precipitate and Cr³⁺ a grey-green precipitate that dissolves in excess to give a green solution. Learn each colour and whether the precipitate redissolves in excess. Write charges with superscript signs, for example NH₄⁺ and Cr³⁺.
Your focus
- Identify from the specification which ion tests are required in 5.3.2.
- Describe each required test as reagent, observation and ionic equation.
- Check a revision list against the specification statements for this section.
Show all 12 objectives
- Carry out test-tube reactions to identify cations and anions in an unknown compound.
- Interpret precipitate colours and solubility in excess reagent to name the ion present.
- Write ionic equations, including complex ion formation, that account for the observations.
- Describe the test and observation for each listed anion
- Write balanced ionic equations for the precipitation reactions
- Select the correct anion from a described observation
- Describe the test and observation for each listed cation
- Explain the behaviour of the chromium(III) precipitate in excess sodium hydroxide
- Select the correct cation from a described observation
Transition elements - 5.3.2 Qualitative analysis exam tips
Marking Points
- Selects aqueous sodium hydroxide and aqueous ammonia as the reagents for identifying cations in solution.
- Records the colour of the precipitate formed and whether it dissolves when excess reagent is added.
- Identifies halide ions by acidifying with dilute nitric acid and adding silver nitrate, noting the precipitate colour.
- Identifies sulfate ions by adding barium chloride or barium nitrate with dilute acid to give a white precipitate.
- Writes the ionic equation for a precipitation or for complex ion formation that matches the stated observation.
- Explains that acidification prevents precipitation of carbonate or hydroxide ions that would confuse the anion test.
- Identifies carbonate by the effervescence of carbon dioxide when dilute acid is added, confirmed by limewater turning milky
- States that silver nitrate with dilute nitric acid gives a white precipitate for chloride, a cream precipitate for bromide and a pale yellow precipitate for iodide
- Describes the sulfate test as adding dilute hydrochloric acid then barium chloride solution to form a white precipitate of barium sulfate
- Writes balanced ionic equations for the precipitation reactions, for example Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
- Uses correct superscript charges such as CO₃²⁻, SO₄²⁻ and Cl⁻ throughout
- Identifies ammonium by warming with sodium hydroxide solution and testing the evolved ammonia with damp red litmus paper turning blue
- States the hydroxide precipitate colours: blue for Cu²⁺, green for Fe²⁺, reddish-brown for Fe³⁺, pale pink for Mn²⁺ and grey-green for Cr³⁺
- Notes that the chromium(III) hydroxide precipitate dissolves in excess sodium hydroxide to give a green solution
- Writes balanced ionic equations for the precipitation reactions, for example Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)
- Uses correct superscript charges such as NH₄⁺, Cu²⁺ and Cr³⁺ throughout
Examiner Tips
- 💡Read the numbered sub-statements beneath the heading and turn each one into a three-part card: reagent, observation, ionic equation.
- 💡Practise writing observations precisely, naming the precipitate colour and any gas with its test result.
- 💡Check your notes against the specification wording before revising, so you do not carry in ions from other sections.
- 💡Learn a decision tree: cation tests with hydroxide and ammonia, halide tests with acidified silver nitrate, sulfate tests with acidified barium salt.
- 💡Practise writing ionic equations from observations, including the tetraamminecopper(II) ion, so equation and colour are linked.
- 💡Read each question for the words 'few drops' or 'excess' and answer only what that stage shows.
- 💡Learn each anion as a reagent–observation–equation triple so any part can be asked
- 💡Quote precipitate colours precisely, since vague colour words lose credit
- 💡Practise writing the three silver halide equations and the barium sulfate equation until they balance
- 💡Build a table of cation, reagent, precipitate colour and behaviour in excess
- 💡Quote colours precisely, since vague colour words lose credit
- 💡Practise balancing the precipitation equations, remembering the 3OH⁻ for Fe³⁺ and Cr³⁺
Common Mistakes
- Assumes the heading lists every cation and anion test in the course; check the numbered sub-statements and learn only the ions they name.
- Records a colour change without naming the reagent and conditions; write the test as reagent, observation and equation together.
- Treats a textbook table of extra ions as examinable content; match your revision list to the specification statements for this section.
- Adds excess reagent before recording the initial precipitate; record the few-drops observation first, then the excess result.
- Omits acidification before the silver nitrate test; add dilute nitric acid first so carbonate ions do not give a misleading precipitate.
- Writes formulae with postfix charge notation; use Unicode superscript signs such as Na⁺, O²⁻ and Cu²⁺.
- States a colour without naming the ion responsible; link each observation to the ion and, where required, the ionic equation.
- Omits the dilute nitric acid before silver nitrate; add it to remove carbonate ions that would also give a precipitate
- Confuses the silver halide colours; keep white for chloride, cream for bromide and pale yellow for iodide
- Uses barium nitrate without acid or forgets the acid; add dilute hydrochloric acid first so carbonate does not interfere
- Writes charges as postfix numbers; use superscript signs such as SO₄²⁻ and Cl⁻
- Confuses the green Fe²⁺ precipitate with the grey-green Cr³⁺ precipitate; distinguish them by the chromium precipitate dissolving in excess
- Forgets that only the chromium(III) precipitate redissolves in excess sodium hydroxide; state this explicitly
- Tests ammonium without warming; warm the mixture so ammonia is released
- Writes charges as postfix numbers; use superscript signs such as NH₄⁺ and Fe³⁺