Four solutions contain Al3+, Fe2+, Cu2+ and Fe3+. State the precipitate colour after adding aqueous sodium hydroxide dropwise to each, and the further observation that confirms Al3+.
[5 marks]
Total for this question: 5
A-level Chemistry required practicals
Identifying aqueous transition-metal ions
AQA 3.2.6 · RP11
A-level Chemistry (7405) · Required practical 11 — 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 solutions containing the specified Al3+, Fe2+, Cu2+ and Fe3+ aqua ions from test-tube reactions and equations (Al3+ is included in the 7405 metal-aqua-ion set although aluminium is not a transition metal).
Apparatus
- Solutions of the specified metal aqua ions and labelled unknowns
- Aqueous sodium hydroxide, aqueous ammonia and aqueous carbonate reagents
- Test tubes, rack, dropping pipettes and glass rods
- Hot-water bath and test-tube holder
- Eye protection and suitable chemical-waste container
Apparatus & techniques (AT)
AT b · controlled heating
Uses a controlled hot-water bath to heat the retained copper(II) hydroxide mixture until it blackens to copper(II) oxide, and to accelerate the standing observations.
AT d · titration/distillation/reflux/qualitative tests/filtration
Carries out ordered small-scale qualitative tests, including dropwise and excess-reagent stages.
AT k · safe handling
Handles metal-ion and corrosive test reagents safely and segregates contaminated waste.
Safety
Hazard
Sodium hydroxide and ammonia can damage eyes or irritate skin and airways.
Control
Wear eye protection, use drops on a small scale and work in good ventilation.
Hazard
Metal-ion solutions are harmful and hot water can scald.
Control
Use a test-tube holder, keep the bath below boiling and collect all metal-containing mixtures as instructed.
Method
- 1Place equal small fresh portions of each known or unknown solution in clearly labelled test tubes so one reagent cannot contaminate another test.
- 2Add aqueous sodium hydroxide dropwise, record any precipitate colour, then add it in excess and record whether the precipitate dissolves.
- 3Repeat on fresh portions with aqueous ammonia, first dropwise and then in excess; record both precipitate and final solution colours.
- 4Use fresh portions for the specified carbonate test and record precipitate colour and any gas produced rather than relying on a vague statement such as ‘a reaction occurs’.
- 5Retain the specified hydroxide mixtures for the hot-water/standing stage: green iron(II) hydroxide darkens to brown as it oxidises to iron(III) on standing in air, and blue copper(II) hydroxide turns black (copper(II) oxide) on heating in the hot-water bath. Compare unknowns with the known-ion matrix before assigning an identity.
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 fresh-portion, dropwise, excess and hot-water sequence without cross-contamination.
- CPAC 3: Editorial focus: control risks from metal-ion solutions, alkali, ammonia and heated water.
- CPAC 4: Editorial focus: record precise colours, solubility changes and gas observations before identifying an ion.
Results & processing
- Build an ion × reagent matrix covering initial precipitate colour, behaviour in excess reagent, solution colour and gas evolution.
- Use ionic, acid–base and ligand-substitution equations to support each diagnostic observation.
- Treat the white Al(OH)3 precipitate as an intermediate observation: its dissolution in excess hydroxide is confirmatory evidence.
Analysis skills
- Construct and use an ion × reagent observation matrix.
- Write balanced precipitation, amphoteric-dissolution and ligand-substitution equations.
- Distinguish confirmatory evidence from a non-unique first observation.
Uncertainty
Sources
- Cross-contamination between dropping pipettes
- Subjective naming of pale or changing colours
- Unequal reagent excess and observation time
Calculations
- No numerical propagation is appropriate; standardise drops or measured small volumes and observation time for comparisons.
Interpretation
- Qualitative uncertainty is reduced by fresh portions and known-ion controls, not by inventing numerical precision.
- A single white precipitate is not conclusive when more than one species or contamination could produce it.
Exam angles
- Identify unknowns from dropwise and excess-reagent observations.
- Complete ionic or ligand-substitution equations and explain the observation each represents.
- Explain why a confirmatory excess-reagent step is needed after a white precipitate.
Where students lose marks
Identifying Al3+ as soon as a white precipitate forms.
Fix: Add excess sodium hydroxide and record dissolution to a colourless solution before confirming aluminium.
- Examiner report: C3-22 · PDF p. 4
Saying only that a precipitate dissolves in excess ammonia for Cu2+.
Fix: Record the deep-blue solution formed by ligand substitution as well as disappearance of the blue precipitate.
Writing equations that lose charge or omit the species responsible for redissolving.
Fix: Balance atoms and charge, then connect the equation to the exact reagent condition.
Improve the method
- Use separate labelled dropping pipettes and fresh portions to prevent cross-contamination.
- Keep sample and reagent volumes comparable so colour intensity and excess-reagent behaviour can be compared fairly.
- View tubes against a white background and compare unknowns directly with known-ion controls.
Source references
- Specification: CSpec 3.2.6 · PDF p. 48
Try it — exam-style
Medium
ORIGINAL
Easy
ORIGINAL
Describe the observations when aqueous ammonia is added dropwise and then in excess to a Cu2+ solution, and name the process responsible for the final change.
[3 marks]
Total for this question: 3
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
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