Plan test-tube reactions, record reliable observations and identify aqueous ions using linked evidence rather than colour alone.
Design a comparison that can distinguish the candidates
Required practical 11 develops test-tube reactions to identify transition-metal ions in aqueous solution. Here the comparison includes the four specified ions Cu²⁺, Fe²⁺, Fe³⁺ and Al³⁺. Use a separate fresh portion for each reagent and label every test tube. Record the initial solution colour before adding anything.
Add reagent dropwise with gentle mixing, record the first change, then continue until it is clearly in excess and record any further change. Check again after standing where oxidation may occur. Use clean pipettes without touching other solutions, clean test tubes and similar sample volumes so cross-contamination or different dilution does not create misleading results.
Use the laboratory’s stated reagent concentrations and risk assessment: eye protection, small quantities and appropriate handling of corrosive alkali and ammonia are necessary. Collect metal-containing waste as instructed. This page teaches interpretation and planning; perform the practical under the laboratory’s supervision.
Record every stage, including no further change
Write “blue precipitate” or “deep-blue solution” as appropriate, not simply “blue”. “Colourless” describes a solution without visible colour; “clear” describes absence of suspended material and can also apply to a coloured solution. “No further visible change in excess” is a useful observation and should not be left blank.
| Ion | NaOH: few drops → excess | NH₃: few drops → excess | Na₂CO₃ |
|---|---|---|---|
| Cu²⁺ | Blue precipitate → remains | Blue precipitate → dissolves, deep-blue solution | Blue-green carbonate precipitate |
| Fe²⁺ | Green precipitate → remains; browns in air | Green precipitate → remains; browns in air | Green carbonate precipitate |
| Fe³⁺ | Brown precipitate → remains | Brown precipitate → remains | Brown hydroxide precipitate + CO₂ |
| Al³⁺ | White precipitate → dissolves, colourless solution | White precipitate → remains | White hydroxide precipitate + CO₂ |
Worked identification: link each conclusion to a test
Constructed unknown A is initially colourless. A white solid forms with a little NaOH and dissolves when excess is added. On a fresh portion, NH₃ gives a white solid that remains in excess. Among the four listed candidates, this pattern identifies Al³⁺. The first test supports amphoteric aluminium hydroxide, and the ammonia result distinguishes excess weak base from excess strong alkali.
Constructed unknown B gives a blue solid with a little NH₃ and a deep-blue solution in excess. This identifies Cu²⁺ among the candidates through the soluble tetraammine–diaqua complex. Confirm on a fresh portion with a blue hydroxide precipitate that remains in excess NaOH.
Constructed unknown C gives a green solid with NaOH which gradually turns brown on standing. Fe²⁺ is initially present; oxygen oxidises the iron(II) hydroxide to iron(III) hydroxide. A fresh Fe³⁺ sample would form a brown precipitate immediately rather than first giving the green stage.
A time-dependent colour change can be redox
In the simplified hydroxide formulae, oxidation by oxygen is 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃. Four Fe(II) centres each lose one electron; O₂ gains four electrons. The precipitate remains a solid but becomes brown as Fe(III) hydroxide develops.
Prepare or obtain fresh Fe²⁺ solutions as directed, record early observations promptly and distinguish “immediate brown precipitate” from “green precipitate that later browns”. Old samples and contaminated reagents can blur the distinction. Do not interpret every delayed colour change as ligand substitution.
Use carbonate when a second line of evidence is needed
Among the four candidates, the combination of precipitate colour and CO₂ formation distinguishes the 3+ aqua-ion reactions from the 2+ carbonate precipitations. A brown precipitate with carbonate plus effervescence supports Fe³⁺; a green precipitate without expected effervescence supports Fe²⁺. Verify CO₂ with limewater if identification of the gas is required.
Effervescence alone cannot identify the metal. Extra acid in the sample would also react with carbonate, and Al³⁺ and Fe³⁺ both release CO₂ in the specified reactions. Equally, one white precipitate is not universally diagnostic outside the supplied candidate set. Name the evidence and state the limits of the inference.
Improve the method by targeting the source of uncertainty
If observations disagree, first check reagent identity, freshness, concentration, contamination and whether excess reagent was actually reached. Repeat with fresh separate portions and compare against known reference solutions under the same conditions. Observe against a white background with good lighting, especially when distinguishing pale solutions or fine precipitates.
Adding every reagent to the same tube destroys the original test conditions: NaOH changes pH, NH₃ changes ligands, and acid can dissolve solids or produce carbonate bubbles. Separate portions let each observation be attributed to the intended reagent. A positive result from a clean control also helps distinguish an absent ion from a failed reagent.
AQA’s practical handbook includes an exemplar with additional tests and heating; its exact experimental sequence is not the only way to address RP11. These notes focus on the four-ion chemistry specified in 3.2.6. Record what actually happens rather than substituting the expected table if your experimental observation differs.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Why should NaOH, NH₃ and carbonate tests use separate portions of an unknown?Show answer
A previous reagent changes pH, ligands or solid products and can alter later reactions. Fresh portions allow the observed change to be linked to the intended test and reduce ambiguous results.
Q2. A white precipitate dissolves in excess NaOH but remains in excess NH₃. Identify the ion among Cu²⁺, Fe²⁺, Fe³⁺ and Al³⁺.Show answer
Al³⁺. Its hydroxide is amphoteric and dissolves in excess strong alkali to [Al(OH)₄]⁻, but remains in excess ammonia under these conditions.
Q3. Why is “the solution went brown” an inadequate record for Fe²⁺ with NaOH on standing?Show answer
It omits the initial green precipitate and fails to distinguish a solid from the solution. Record a green precipitate that becomes brown on standing as Fe(II) hydroxide is oxidised to Fe(III) hydroxide.
Q4. A sample bubbles with carbonate. Is this enough to identify Fe³⁺?Show answer
No. Al³⁺ aqua ions can also release CO₂, and additional acid can cause carbonate effervescence. Use precipitate colour, the other reagent tests and the stated candidate set; confirm the gas if required.
Q5. A student adds one drop of ammonia, sees a blue precipitate and concludes it cannot dissolve. What is missing?Show answer
They have tested limited ammonia only. They must add a sufficient excess with mixing and record whether the solid dissolves to form the deep-blue copper ammine solution.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 inorganic chemistry specification — 3.2.6 coverage and required skills.
- Chemrevise: Reactions of ions in aqueous solution — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 1 mark scheme — Q07.2–07.6 pp23–24: aqua-ion formulae, charge density, O–H polarisation, ammonia equation and aluminium precipitate; report Q07 p5.
- AQA June 2023 Paper 1 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
- AQA practical handbook: required practical 11 — Printed pp150–155: planning, separate test observations, dropwise/excess additions and changes on standing. The original teaching examples here are not copied experimental results.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.