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A-level Chemistry required practicals

Test-tube identification of inorganic ions

AQA 3.2.3 · RP4

A-level Chemistry (7405) · Required practical 4 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.

Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works

Identify selected Group 2 and ammonium cations and halide, hydroxide, carbonate and sulfate anions using a logical sequence of small-scale tests.

Apparatus

  • Clean test tubes, rack and dropping pipettes
  • Aqueous sodium hydroxide and damp red litmus paper
  • Dilute sulfuric acid for a Group 2 sulfate-solubility comparison
  • Dilute nitric acid, silver nitrate and dilute or concentrated ammonia
  • Dilute acid and limewater for carbonate testing
  • Acidified BaCl2 solution for sulfate testing
  • Indicator paper or solution for hydroxide testing

Apparatus & techniques (AT)

AT d · titration/distillation/reflux/qualitative tests/filtration

Carry out small-scale qualitative tests on fresh portions, controlling reagent order and excess addition before assigning an ion.

AT k · safe handling

Handle corrosive, oxidising, harmful and toxic reagents safely while avoiding cross-contamination.

Safety

Hazard

Sodium hydroxide, acids and ammonia solutions can damage eyes and irritate skin or airways.

Control

Wear eye protection, use drop-scale quantities, warm gently and avoid inhaling directly from the test tube.

Hazard

Barium chloride is toxic; silver nitrate is harmful and also stains skin and clothing.

Control

Use small quantities, avoid contact and collect silver- and barium-containing waste as instructed rather than pouring it away indiscriminately.

Method

  1. 1Divide the unknown into labelled fresh portions so one reagent cannot interfere with a later test.
  2. 2For a Group 2 cation, add sodium hydroxide to a fresh portion to compare hydroxide precipitation, then add dilute sulfuric acid to another fresh portion to compare sulfate precipitation.
  3. 3For ammonium ions, add aqueous sodium hydroxide, warm gently and test any gas with damp red litmus paper; ammonia turns it blue.
  4. 4For carbonate ions, add dilute acid and pass any gas produced into limewater; carbon dioxide turns the limewater cloudy.
  5. 5For sulfate ions, add acidified BaCl2 solution to a fresh portion; a persistent white BaSO4 precipitate supports sulfate.
  6. 6For halides, acidify a fresh portion with dilute nitric acid, add silver nitrate and then test precipitate solubility in ammonia where distinction is needed.
  7. 7For hydroxide ions, test a fresh portion with damp red litmus paper (turns blue) or universal indicator (blue/purple) and record the observation precisely.
  8. 8Record reagent, condition and observation before using the combined evidence to identify each ion.

CPAC focus (editorial)

This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.

  • CPAC 1: Editorial focus: follow the test sequence and reagent conditions exactly enough to prevent one test invalidating another.
  • CPAC 3: Editorial focus: manage corrosive reagents and toxic-metal waste hazards during small-scale testing.
  • CPAC 4: Editorial focus: record colour, precipitate, gas test and excess-reagent behaviour before naming the unknown.

Results & processing

  • Build an observation matrix with separate columns for reagent, condition, initial observation, excess-reagent behaviour and inference.
  • With sodium hydroxide, Mg2+ gives a white precipitate because Mg(OH)2 is sparingly soluble; hydroxide solubility increases down the group, so Ba2+ gives no precipitate.
  • With dilute sulfuric acid, Ba2+ gives a white BaSO4 precipitate while Mg2+ gives none (MgSO4 is soluble) — sulfate solubility decreases down the group, the opposite trend to the hydroxides.
  • Warming an ammonium salt with aqueous hydroxide ions releases ammonia gas, which turns damp red litmus paper blue.
  • Use confirmatory evidence rather than identification by elimination alone; distinguish the observation from the ion conclusion.
  • Write ionic equations with charges and state symbols for precipitate formation, gas evolution or neutralisation when requested.

Analysis skills

  • Construct a minimum decision sequence that distinguishes the candidate ions without allowing earlier reagents to contaminate later portions.
  • Link each recorded observation to a confirmatory inference and a balanced ionic equation where applicable.
  • Explain that BaCl2 solution is used because BaSO4 is insoluble, giving the white precipitate; acidify with dilute HCl or nitric acid — never sulfuric acid, which would introduce sulfate — to remove carbonate and sulfite interference first.

Uncertainty

Sources

  • Carry-over on dropping pipettes or reused test tubes can create false precipitates or misleading precipitate colours.
  • Weak precipitates, mixed unknowns and subjective descriptions such as cream or pale yellow can blur distinctions.

Calculations

  • This diagnostic practical normally uses no numerical uncertainty calculation; evaluation instead tracks false-positive and false-negative routes through the test sequence.

Interpretation

  • A negative test is useful only when the correct reagent, condition and sufficient amount were used.
  • A single similar-looking white precipitate is rarely enough: fresh portions and confirmatory behaviour provide stronger identification.

Exam angles

  • Design the minimum sequence needed to identify an unknown from a stated candidate list.
  • State exact reagents, conditions and positive observations rather than giving an ion name alone.
  • Explain acidification in sulfate or halide testing and complete ionic equations with correct state symbols.
  • Distinguish visually similar precipitates using a valid confirmatory test or excess-reagent behaviour.

Where students lose marks

Testing for ammonium ions with sodium hydroxide but not warming, or reporting ammonia without a gas test.

Fix: Warm gently and show that the evolved gas turns damp red litmus paper blue.

  • Examiner report: C3-22 · PDF p. 4

Adding BaCl2 solution without first acidifying the sample.

Fix: Use acidified BaCl2 so carbonate and sulfite are removed and cannot form misleading white barium precipitates.

  • Examiner report: C3-22 · PDF p. 4

Naming an ion from a colour or precipitate without recording the reagent, condition or excess-reagent result.

Fix: Record the complete observation first, then use a confirmatory test or comparison before stating the identity.

Improve the method

  • Use clean labelled pipettes for each reagent and fresh portions of unknown for independent tests.
  • Compare weak colours with known standards under the same lighting, while avoiding absolute identification from colour alone.
  • Plan the shortest discriminating sequence before testing so hazardous reagents and sample are not used unnecessarily.

Source references

  • Specification: CSpec 3.2.3 · PDF p. 41
  • Specification: CSpec 3.2.3 · PDF p. 42

Try it — exam-style

Medium
ORIGINAL

An unknown solution releases a gas that turns damp red litmus paper blue when warmed with aqueous NaOH. A fresh portion gives a cream precipitate after dilute HNO3 and AgNO3 are added. Identify both ions and write the two ionic equations.

[4 marks]

Total for this question: 4

Hard
ORIGINAL

Two colourless solutions contain CO32− and SO42−, one ion in each. Describe a sequence that identifies both and explain why the sulfate-test portion is acidified.

[4 marks]

Total for this question: 4

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

A-level Chemistry specification coverage

Practise Group 2, qualitative analysis and every AQA 7405 specification point.

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