1. Acidity of Aqueous Metal Ions (2+ vs 3+)
Transition metal ions and aluminium ions dissolve in water to form octahedral hexaaqua complex ions: [M(H₂O)₆]²⁺ or [M(H₂O)₆]³⁺.
- 2+ ions ([Fe(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺): Form weakly acidic solutions with pH ~ 5 to 6.
- 3+ ions ([Fe(H₂O)₆]³⁺, [Al(H₂O)₆]³⁺): Form distinctly acidic solutions with pH ~ 2 to 3.
Polarisation & Charge Density Mechanism
A 3+ metal ion has a higher ionic charge and a smaller ionic radius than a 2+ ion, resulting in a substantially higher charge density.
- The small, highly charged 3+ cation strongly attracts electron density from the coordinate bonds toward itself.
- This polarises the O−H bonds within the coordinated water ligands, pulling electron density away from hydrogen.
- The O−H bond is significantly weakened, enabling water molecules from the surrounding solvent to act as Brønsted-Lowry bases and remove a proton:
[M(H₂O)₆]³⁺(aq) + H₂O(l) ⇌ [M(H₂O)₅(OH)]²⁺(aq) + H₃O⁺(aq) - Because 2+ ions have lower charge density, they polarise O−H bonds much less effectively, releasing far fewer protons into solution.
Related: test for these ions in RP9, test-tube reactions of transition metal ions. For the oxidation of Fe²⁺ and other redox changes, see Variable Oxidation States and Catalysts.
2. Precipitation Reactions with Dilute Sodium Hydroxide (NaOH)
Adding aqueous hydroxide ions (OH⁻) progressively removes protons from coordinated water ligands until an uncharged, insoluble neutral metal hydroxide precipitates:
| Metal Ion | Initial Solution | Precipitate with Dilute NaOH | Ionic Equation for Precipitation | Effect of Excess NaOH |
|---|---|---|---|---|
| Fe²⁺ | Pale green | Green precipitate of Fe(H₂O)₄(OH)₂(s) | [Fe(H₂O)₆]²⁺ + 2OH⁻ → Fe(H₂O)₄(OH)₂(s) + 2H₂O | Insoluble in excess. (Darkens to brown on standing as air oxidises Fe²⁺ to Fe³⁺). |
| Cu²⁺ | Pale blue | Blue precipitate of Cu(H₂O)₄(OH)₂(s) | [Cu(H₂O)₆]²⁺ + 2OH⁻ → Cu(H₂O)₄(OH)₂(s) + 2H₂O | Insoluble in excess. |
| Fe³⁺ | Yellow / brown | Brown precipitate of Fe(H₂O)₃(OH)₃(s) | [Fe(H₂O)₆]³⁺ + 3OH⁻ → Fe(H₂O)₃(OH)₃(s) + 3H₂O | Insoluble in excess. |
| Al³⁺ | Colourless | White precipitate of Al(H₂O)₃(OH)₃(s) | [Al(H₂O)₆]³⁺ + 3OH⁻ → Al(H₂O)₃(OH)₃(s) + 3H₂O | DISSOLVES in excess to form a colourless solution of [Al(OH)₄]⁻(aq). |
3. Amphoteric Dissolution of Aluminium Hydroxide
Aluminium hydroxide, Al(H₂O)₃(OH)₃, is amphoteric. It behaves as a base by accepting protons in acid, and as an acid by donating a proton to excess hydroxide:
Acting as a base: with acid
Al(H₂O)₃(OH)₃(s) + 3H⁺(aq) → [Al(H₂O)₆]³⁺(aq)
The hydroxide ligands accept protons and become water ligands again. The white precipitate dissolves to give a colourless solution.
Acting as an acid: with excess hydroxide
Al(H₂O)₃(OH)₃(s) + OH⁻(aq) → [Al(OH)₄(H₂O)₂]⁻(aq) + H₂O(l)
A water ligand loses a proton to the OH⁻ ion. This equation is also often written without the water ligands:
Al(H₂O)₃(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq) + 3H₂O(l)
The white precipitate re-dissolves in excess NaOH to yield a completely colourless solution of sodium tetrahydroxoaluminate.
4. Reactions with Dilute & Excess Aqueous Ammonia (NH₃)
Aqueous ammonia establishes an equilibrium: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. In dilute ammonia, OH⁻ acts as a Brønsted-Lowry base, producing the exact same hydroxide precipitates as dilute NaOH.
Precipitation equations with ammonia
NH₃ acts as a base. It removes protons from water ligands, and NH₄⁺ ions form.
| Metal ion | Equation | Observation |
|---|---|---|
| Fe²⁺ | [Fe(H₂O)₆]²⁺ + 2NH₃ → Fe(H₂O)₄(OH)₂ + 2NH₄⁺ | Green precipitate |
| Cu²⁺ | [Cu(H₂O)₆]²⁺ + 2NH₃ → Cu(H₂O)₄(OH)₂ + 2NH₄⁺ | Blue precipitate (dissolves in excess, see below) |
| Fe³⁺ | [Fe(H₂O)₆]³⁺ + 3NH₃ → Fe(H₂O)₃(OH)₃ + 3NH₄⁺ | Brown precipitate |
| Al³⁺ | [Al(H₂O)₆]³⁺ + 3NH₃ → Al(H₂O)₃(OH)₃ + 3NH₄⁺ | White precipitate. It does not dissolve in excess NH₃. |
Behaviour with Excess Ammonia: Ligand Substitution in Cu²⁺
When excess concentrated ammonia is added to copper(II) hydroxide, ammonia molecules act as Lewis base ligands and displace water molecules in a ligand substitution reaction:
Cu(H₂O)₄(OH)₂(s) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2H₂O(l) + 2OH⁻(aq)
The blue precipitate dissolves completely to yield an intense, deep royal blue solution of tetraamminediaquacopper(II).
In contrast, the precipitates of Fe²⁺, Fe³⁺, and Al³⁺ do NOT dissolve in excess ammonia.
5. Precipitate Colour Rack Diagram
6. Reactions with Sodium Carbonate (Na₂CO₃)
The reaction of aqueous metal ions with sodium carbonate (Na₂CO₃) illustrates the profound chemical difference between 2+ and 3+ ions:
| Ion Type | Reaction Behaviour with Na₂CO₃ | Observations | Balanced Ionic Equation |
|---|---|---|---|
| 2+ Ions (Fe²⁺, Cu²⁺) | Simple precipitation of insoluble metal carbonate. No gas formed. |
Fe²⁺: Green precipitate of FeCO₃(s).
Cu²⁺: Blue-green precipitate of CuCO₃(s). |
Fe²⁺(aq) + CO₃²⁻(aq) → FeCO₃(s)
Cu²⁺(aq) + CO₃²⁻(aq) → CuCO₃(s) |
| 3+ Ions (Fe³⁺, Al³⁺) | Acid-base reaction! 3+ ions are too acidic to form carbonates. Protons released react with CO₃²⁻ to produce CO₂ gas and precipitate the metal hydroxide. |
Fe³⁺: Brown precipitate of Fe(OH)₃ and effervescence (bubbles of CO₂).
Al³⁺: White precipitate of Al(OH)₃ and effervescence (bubbles of CO₂). |
2[Fe(H₂O)₆]³⁺ + 3CO₃²⁻ → 2Fe(H₂O)₃(OH)₃(s) + 3CO₂(g) + 3H₂O(l)
2[Al(H₂O)₆]³⁺ + 3CO₃²⁻ → 2Al(H₂O)₃(OH)₃(s) + 3CO₂(g) + 3H₂O(l) |
- M²⁺ ions give MCO₃ precipitates: FeCO₃ is green and CuCO₃ is blue-green.
- M³⁺ ions give M(OH)₃ precipitates and CO₂ fizzing, because [M(H₂O)₆]³⁺ is acidic enough to react with carbonate ions.
Never write Fe₂(CO₃)₃ or Al₂(CO₃)₃ as products! Because hexaaqua 3+ ions are strongly acidic (pH 2 to 3), carbonate ions are protonated to hydrogencarbonate and then decompose into carbon dioxide and water. The products are ALWAYS the metal hydroxide precipitate and carbon dioxide gas.
7. Worked Calculations & Identification Schemes
Test 1: Addition of aqueous sodium hydroxide (NaOH):
- If Fe²⁺: A green precipitate of Fe(H₂O)₄(OH)₂ forms, which darkens to brown at the surface upon standing in air.
- If Fe³⁺: A red-brown precipitate of Fe(H₂O)₃(OH)₃ forms immediately.
Test 2: Addition of aqueous sodium carbonate (Na₂CO₃):
- If Fe²⁺: A green precipitate of FeCO₃ forms with no bubbles of gas.
- If Fe³⁺: A brown precipitate of Fe(H₂O)₃(OH)₃ forms alongside vigorous effervescence (bubbles of CO₂ gas) that turns limewater cloudy.
Stage 1: Precipitation with dilute ammonia:
[Cu(H₂O)₆]²⁺(aq) + 2NH₃(aq) → Cu(H₂O)₄(OH)₂(s) + 2NH₄⁺(aq)
Stage 2: Dissolution in excess concentrated ammonia:
Cu(H₂O)₄(OH)₂(s) + 4NH₃(aq) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2H₂O(l) + 2OH⁻(aq)
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
Exam-Style Practice Questions
Test your understanding of these core syllabus concepts with targeted questions and detailed explanations.
Question 1: Why is an aqueous solution of iron(III) chloride significantly more acidic (pH ~2) than an aqueous solution of iron(II) chloride (pH ~5)?
Show Answer & Explanation
Correct Answer: B
Fe³⁺ has a higher charge and smaller ionic radius than Fe²⁺, giving it higher charge density. This polarises the O−H bonds of water ligands, weakening them and releasing H⁺ ions into solution.
Question 2: Which metal hydroxide precipitate dissolves in excess sodium hydroxide to form a colourless solution?
Show Answer & Explanation
Correct Answer: D
Al(OH)₃ is amphoteric. It reacts with excess OH⁻ to form the soluble tetrahydroxoaluminate ion, [Al(OH)₄]⁻, producing a completely colourless solution.
Question 3: What is observed when aqueous sodium carbonate is added to an aqueous solution of aluminium chloride?
Show Answer & Explanation
Correct Answer: B
[Al(H₂O)₆]³⁺ is strongly acidic. It undergoes an acid-base reaction with carbonate ions (CO₃²⁻) to form carbon dioxide gas (bubbles) and precipitates aluminium hydroxide: 2[Al(H₂O)₆]³⁺ + 3CO₃²⁻ → 2Al(H₂O)₃(OH)₃(s) + 3CO₂(g) + 3H₂O(l).
Question 4: What colour change is observed when excess concentrated ammonia is added to a blue precipitate of copper(II) hydroxide?
Show Answer & Explanation
Correct Answer: B
Copper(II) hydroxide undergoes ligand exchange with excess ammonia to form the soluble [Cu(NH₃)₄(H₂O)₂]²⁺ complex ion, which has a characteristic intense deep royal blue colour.
Written Questions
Write your answer, then open the mark scheme. Each point marked (1) is worth one mark.
Question 5 (2 marks): Write an equation for the reaction of [Fe(H₂O)₆]³⁺ with aqueous ammonia. Give the observation.
Show Answer & Mark Scheme
- [Fe(H₂O)₆]³⁺ + 3NH₃ → Fe(H₂O)₃(OH)₃ + 3NH₄⁺ (1)
- Brown precipitate (1)
Question 6 (2 marks): Aluminium hydroxide is amphoteric. Write an equation for its reaction with H⁺ ions and an equation for its reaction with OH⁻ ions.
Show Answer & Mark Scheme
- Al(H₂O)₃(OH)₃ + 3H⁺ → [Al(H₂O)₆]³⁺ (1)
- Al(H₂O)₃(OH)₃ + OH⁻ → [Al(OH)₄(H₂O)₂]⁻ + H₂O (1)
Question 7 (3 marks): Sodium carbonate solution is added to separate solutions of iron(II) sulfate and iron(III) chloride. Describe what you would see in each case, and explain the difference.
Show Answer & Mark Scheme
- Fe²⁺: green precipitate (of FeCO₃) (1)
- Fe³⁺: brown precipitate and bubbles of gas (CO₂) (1)
- [Fe(H₂O)₆]³⁺ is more acidic, so it reacts with carbonate ions to give CO₂ and the hydroxide (1)