AQA A-Level Chemistry: Reactions of ions in aqueous solution (7405) – Revision Notes

These notes condense section 3.2.6, Reactions of ions in aqueous solution, of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams from June 2016 onwards. The whole section is marked “A-level only”. Inorganic chemistry is listed under A-level Paper 1, and Paper 3 can assess any content. Required practical 11 belongs to this section.

For full explanations and worked examples, use the study guide. Then test yourself with the practice questions. Ligand substitution in general is in transition metals. For the whole course, see the AQA A-Level Chemistry hub, the printable checklist and the free diagnostics.

Scope (3.2.6, A-level only)

• 2+ aqua ions: Fe and Cu only.
• 3+ aqua ions: Al and Fe only.
• Bases: OH⁻, NH₃ and CO₃²⁻ only.
• Explain acidity using charge/size ratio.
• Amphoteric hydroxide example: aluminium.
• Required practical 11: simple test-tube reactions to identify transition metal ions in aqueous solution.

Key definitions

• Metal-aqua ion: a metal ion with six water ligands, each bonded by a co-ordinate bond from oxygen, for example [Cu(H₂O)₆]²⁺.
• Acidity of an aqua ion: its tendency to give H⁺ from a water ligand to a solvent water molecule.
• Amphoteric: able to react with (dissolve in) both acids and bases.
• Charge/size ratio: ionic charge compared with ionic radius; a higher value means a more polarising ion.

Colours of the aqua ions

Ion	Colour of solution
[Fe(H₂O)₆]²⁺	pale green
[Cu(H₂O)₆]²⁺	blue
[Fe(H₂O)₆]³⁺	yellow-brown (as usually seen)
[Al(H₂O)₆]³⁺	colourless

Acidity in four steps

[M(H₂O)₆]³⁺ + H₂O ⇌ [M(H₂O)₅(OH)]²⁺ + H₃O⁺

1. M³⁺ has a higher charge and a smaller radius than M²⁺.
2. So it has a higher charge/size ratio and is more polarising.
3. It pulls electron density from the O of the water ligands, weakening the O–H bonds.
4. H⁺ is released more easily: [M(H₂O)₆]³⁺ solutions are more acidic (lower pH) than [M(H₂O)₆]²⁺ solutions of the same concentration.

Master observation table

Ion	NaOH, few drops	NaOH, excess	NH₃, few drops	NH₃, excess	Na₂CO₃
Fe²⁺	green ppt	no change	green ppt	no change	green ppt (FeCO₃)
Cu²⁺	pale blue ppt	no change	pale blue ppt	deep blue solution	blue-green ppt (CuCO₃)
Fe³⁺	brown ppt	no change	brown ppt	no change	brown ppt + bubbles
Al³⁺	white ppt	colourless solution	white ppt	no change	white ppt + bubbles

ppt = precipitate. The iron(II) precipitate darkens to brown on standing in air, as iron(II) is oxidised to iron(III).

Read the table by column for “what’s different”:

• Excess NaOH dissolves only the aluminium precipitate (amphoteric).
• Excess NH₃ dissolves only the copper precipitate (ligand substitution).
• Carbonate gives bubbles only with the 3+ ions.

Formulas of the products

Product	Formula
iron(II) hydroxide	[Fe(H₂O)₄(OH)₂]
copper(II) hydroxide	[Cu(H₂O)₄(OH)₂]
iron(III) hydroxide	[Fe(H₂O)₃(OH)₃]
aluminium hydroxide	[Al(H₂O)₃(OH)₃]
aluminate ion	[Al(H₂O)₂(OH)₄]⁻, also written [Al(OH)₄]⁻
copper–ammonia complex	[Cu(NH₃)₄(H₂O)₂]²⁺
carbonates (2+ only)	FeCO₃, CuCO₃

The neutral hydroxides carry no charge. That is why they precipitate.

Method in steps: writing any equation in this section

1. Start from the hexaaqua ion, [M(H₂O)₆]ⁿ⁺.
2. Decide how many H⁺ the base removes: 2 for M²⁺, 3 for M³⁺ to reach the neutral hydroxide.
3. Pick the H⁺ acceptor:
   • OH⁻ → H₂O
   • NH₃ → NH₄⁺
   • CO₃²⁻ → CO₂ + H₂O (3+ ions only)
4. Write the neutral hydroxide with (6 − n) H₂O ligands and n OH⁻ ligands.
5. Check atoms and charge.

OH⁻:  [M(H₂O)₆]²⁺ + 2OH⁻ → [M(H₂O)₄(OH)₂] + 2H₂O
NH₃:  [M(H₂O)₆]³⁺ + 3NH₃ → [M(H₂O)₃(OH)₃] + 3NH₄⁺
CO₃²⁻ (2+): [M(H₂O)₆]²⁺ + CO₃²⁻ → MCO₃ + 6H₂O
CO₃²⁻ (3+): 2[M(H₂O)₆]³⁺ + 3CO₃²⁻ → 2[M(H₂O)₃(OH)₃] + 3CO₂ + 3H₂O

The three extra equations to learn:

[Al(H₂O)₃(OH)₃] + OH⁻ → [Al(H₂O)₂(OH)₄]⁻ + H₂O
[Al(H₂O)₃(OH)₃] + 3H⁺ → [Al(H₂O)₆]³⁺
[Cu(H₂O)₄(OH)₂] + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2H₂O + 2OH⁻

Must-know distinctions

• NH₃ as a base vs NH₃ as a ligand. A few drops: base (precipitate). Excess with Cu²⁺: ligand (deep blue solution).
• 2+ vs 3+ with carbonate. 2+: metal carbonate precipitates. 3+: hydroxide precipitates and CO₂ is given off, because the 3+ aqua ion is acidic enough to protonate carbonate.
• Amphoteric vs ligand substitution. Al(OH)₃ dissolves in excess NaOH because it loses another H⁺ (acting as an acid). Cu(OH)₂ dissolves in excess NH₃ because NH₃ replaces ligands.
• Colourless vs clear. A solution with no colour is “colourless”. “Clear” means transparent and can be blue.
• Why a precipitate forms vs why it redissolves. A precipitate forms when the complex loses enough H⁺ to become neutral (or, for 2+ ions with carbonate, when MCO₃ forms). It redissolves only by further H⁺ loss (Al) or by ligand substitution (Cu).
• Fe²⁺ vs Fe³⁺. Green vs brown hydroxide; no bubbles vs bubbles with carbonate.

Required practical 11 in brief

1. About 1 cm³ of test solution in a clean test tube; record its colour.
2. Add reagent drop by drop, shaking; record precipitate colour.
3. Continue to excess; record whether the precipitate dissolves.
4. Use a fresh sample for each reagent (NaOH, NH₃, Na₂CO₃).
5. Leave an iron(II) precipitate to stand and look for darkening.

Hazards: NaOH(aq) is corrosive; NH₃(aq) is an irritant. Wear eye protection and work in a ventilated room.

The specification also suggests (as an opportunity, not required content) that you identify the positive and negative ions met elsewhere in the course. The Group 2 cation tests and sulfate test are in Group 2 revision notes.

Small worked reminders

Moles of base. 0.0200 mol of [Fe(H₂O)₆]³⁺ needs 3 × 0.0200 = 0.0600 mol NH₃ to precipitate all of it as [Fe(H₂O)₃(OH)₃].

Gas from carbonate. 0.0100 mol of [Al(H₂O)₆]³⁺ with excess carbonate: Al³⁺ : CO₂ = 2 : 3, so n(CO₂) = 0.0100 × 3/2 = 0.0150 mol.

Quick self-test

1. Give the formula and colour of the copper(II) aqua ion.
2. Which is more acidic: [Fe(H₂O)₆]²⁺ or [Fe(H₂O)₆]³⁺?
3. Give the two features of the 3+ ion that give it a higher charge/size ratio.
4. Write the formula of the precipitate formed when a few drops of NaOH(aq) are added to Fe³⁺(aq).
5. What do you see when excess NaOH(aq) is added to Al³⁺(aq)?
6. What do you see when excess NH₃(aq) is added to Cu²⁺(aq)?
7. Write the equation for [Fe(H₂O)₆]²⁺ with sodium carbonate solution.
8. Why does Fe³⁺(aq) fizz with sodium carbonate solution but Fe²⁺(aq) does not?
9. Define amphoteric and name the example in this section.
10. Write the equation for aluminium hydroxide dissolving in acid.
11. A green precipitate forms with NaOH(aq) and turns brown at the surface on standing. Which ion is present?
12. How many moles of OH⁻ react with 0.0100 mol of [Cu(H₂O)₆]²⁺ to form the precipitate?

Answers

1. [Cu(H₂O)₆]²⁺, blue.
2. [Fe(H₂O)₆]³⁺.
3. Higher charge (3+) and smaller ionic radius.
4. [Fe(H₂O)₃(OH)₃].
5. A white precipitate forms, then dissolves in excess to give a colourless solution.
6. A pale blue precipitate forms first, then dissolves to give a deep blue solution.
7. [Fe(H₂O)₆]²⁺ + CO₃²⁻ → FeCO₃ + 6H₂O.
8. [Fe(H₂O)₆]³⁺ is acidic enough to give H⁺ to carbonate ions, forming CO₂; [Fe(H₂O)₆]²⁺ is too weakly acidic, so FeCO₃ precipitates instead.
9. Reacts with both acids and bases; aluminium hydroxide.
10. [Al(H₂O)₃(OH)₃] + 3H⁺ → [Al(H₂O)₆]³⁺.
11. Fe²⁺ (iron(II)).
12. 2 × 0.0100 = 0.0200 mol.

Where marks are usually lost

• Writing “Fe(OH)₃ precipitate” when the question asks for the formula of the complex: give [Fe(H₂O)₃(OH)₃].
• Leaving a charge on a neutral hydroxide, or leaving the charge off [Cu(NH₃)₄(H₂O)₂]²⁺.
• Stating that FeCO₃-type carbonates form for Fe³⁺ or Al³⁺.
• Missing “effervescence” as a separate observation for 3+ ions with carbonate.
• Giving only the final observation with excess reagent and missing the precipitate that forms first.
• Explaining acidity with charge alone; you need both charge and size, then the effect on the O–H bond.
• Saying ammonia dissolves aluminium hydroxide, or NaOH dissolves copper hydroxide.
• Writing “clear” instead of “colourless”.
• Forgetting NH₄⁺ as the product when ammonia acts as a base.

Official syllabus

AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards (AQA). Section 3.2.6 Reactions of ions in aqueous solution (A-level only), including required practical 11.