Compare ammonia as a proton acceptor and ligand, and explain why carbonate gives different products with 2+ and 3+ aqua ions.
A little ammonia accepts protons
With a limited amount of NH₃, each removed proton makes NH₄⁺. The hydrated hydroxide precipitates and their colours match those formed by limited OH⁻, but the equations have different soluble products. Ammonia is acting as a Brønsted–Lowry base here; do not automatically draw an ammine complex for every addition of ammonia.
Excess ammonia distinguishes copper from the other three
In excess NH₃, the blue copper(II) precipitate dissolves to form a deep-blue solution containing [Cu(NH₃)₄(H₂O)₂]²⁺. Here ammonia also acts as a ligand/Lewis base, donating lone pairs to copper. The full complex has four ammonia and two water ligands, coordination number six and Cu oxidation state +2.
Iron(II), iron(III) and aluminium hydroxide precipitates do not dissolve in excess ammonia under the ordinary tests in this topic. In particular, aluminium dissolving in excess NaOH does not imply that it also dissolves in excess NH₃. Ammonia is a weak base and does not provide the same hydroxide concentration as excess strong alkali.
| Ion | Limited NH₃ | Excess NH₃ |
|---|---|---|
| Cu²⁺ | Blue precipitate | Dissolves; deep-blue solution |
| Fe²⁺ | Green precipitate | Remains; can brown on standing |
| Fe³⁺ | Brown precipitate | Remains |
| Al³⁺ | White precipitate | Remains |
Carbonate with 2+ ions gives insoluble carbonates
For the AQA simplified ionic equations, Cu²⁺ gives a blue-green copper(II) carbonate precipitate and Fe²⁺ a green iron(II) carbonate precipitate. No carbon dioxide is expected from the idealised precipitation itself. Carbonate retains its carbon atom in the solid.
Real copper(II) carbonate precipitates often contain basic copper carbonate/hydroxide; the simple CuCO₃ formula is the stipulated model used here. This qualification does not change the required contrast with the acidic 3+ aqua ions. Any additional acid in a sample can also produce CO₂ with carbonate and must be considered.
Carbonate acts as a base towards the more acidic 3+ ions
With Fe³⁺ and Al³⁺ aqua ions, carbonate accepts protons from water ligands. Carbon dioxide and water form, while the hydrated metal hydroxide precipitates. The observations are a brown precipitate plus effervescence for iron(III), or a white precipitate plus effervescence for aluminium.
Do not write Fe₂(CO₃)₃ or Al₂(CO₃)₃ as the expected precipitate in these aqueous tests. The greater acidity of the 3+ aqua ions, arising from high charge density and stronger O–H polarisation, explains why carbonate undergoes protonation rather than simply precipitating an intact metal carbonate.
Two 3+ aqua ions lose six protons altogether to make two neutral trihydroxide precipitates. Three carbonate ions accept those six protons, giving three CO₂ and three water molecules. Use this reasoning to reconstruct the coefficients rather than memorising a long unbalanced line.
Confirm the gas and keep observation separate from explanation
Bubbles are an observation; “carbon dioxide forms” is an identification that needs the known reaction or a confirmatory test. Pass a suitable sample of evolved gas into limewater: formation of a white precipitate/milkiness supports CO₂. Work with fresh portions so added acid from a previous test does not produce misleading bubbles with carbonate.
Original calculation: 0.800 mmol Al³⁺ aqua ions react completely by the carbonate equation, with no extra acid present. The CO₂ amount is (3/2) × 0.800 = 1.20 mmol. At a supplied gas molar volume of 24.0 dm³ mol⁻¹, the ideal gas volume is 0.00120 × 24.0 = 0.0288 dm³ = 28.8 cm³. Dissolution of CO₂ and experimental losses can lower a collected volume.
Use a reaction classification that matches the change
| Situation | Main process in the model | Evidence |
|---|---|---|
| Limited OH⁻ or NH₃ with any of the four ions | Deprotonation followed by precipitation | Neutral hydrated hydroxide solid |
| Excess NH₃ with Cu²⁺ | Complex formation/ligand substitution | Solid dissolves; deep-blue solution |
| Excess OH⁻ with Al³⁺ hydroxide | Amphoteric dissolution and hydroxo-complex formation | Solid dissolves; colourless solution |
| Carbonate with Cu²⁺ or Fe²⁺ | Carbonate precipitation | Solid without expected gas |
| Carbonate with Fe³⁺ or Al³⁺ | Acid–base reaction plus hydroxide precipitation | Solid and CO₂ effervescence |
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Why do limited NH₃ and limited OH⁻ give the same precipitate but different balanced equations?Show answer
Both remove protons from coordinated water. NH₃ becomes NH₄⁺ whereas OH⁻ becomes H₂O. The neutral metal hydroxide product can be the same while the other products differ.
Q2. An initially blue precipitate dissolves in excess ammonia. Identify the likely complex and its coordination number.Show answer
[Cu(NH₃)₄(H₂O)₂]²⁺ gives the deep-blue solution. Four ammonia plus two water donor bonds give coordination number six.
Q3. What do Fe³⁺ and Fe²⁺ solutions each produce on adding carbonate in the specified tests?Show answer
Fe³⁺ gives brown hydrated iron(III) hydroxide and CO₂ bubbles. Fe²⁺ gives a green FeCO₃ precipitate without expected gas from the ideal precipitation reaction.
Q4. Calculate the ideal moles of CO₂ from 0.00300 mol Fe³⁺ aqua ions with sufficient carbonate and no extra acid.Show answer
Two Fe³⁺ aqua ions give three CO₂. n(CO₂) = (3/2) × 0.00300 = 0.00450 mol.
Q5. Why is aluminium carbonate an unsuitable product for the Al³⁺ aqua-ion test?Show answer
The strongly polarising Al³⁺ centre makes its water ligands sufficiently acidic to protonate carbonate. Hydroxide precipitates while CO₂ and water form; an intact Al₂(CO₃)₃ precipitate is not the expected aqueous product.
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.