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Learn9701Paper 4 · A Level Structured QuestionsChemistry of transition elementsReactions of aqua ions
Chemistry of transition elements · Subtopic 3 of 3

Reactions of Aqua Ions

Metal ions in water exist as hexaaqua ions [M(H₂O)₆]ⁿ⁺. 3+ ions are more acidic than 2+: higher charge density polarises the O–H bonds in water ligands, releasing H⁺. With OH⁻ or a little NH₃, ions are deprotonated to neutral hydroxide precipitates: Cu²⁺ blue; Fe²⁺ green (turns brown in air); Fe³⁺ orange-brown; Cr³⁺ grey-green; Al³⁺ white; Mn²⁺ off-white (browns). Excess NaOH dissolves amphoteric Al(OH)₃ and Cr(OH)₃. Excess NH₃ causes ligand exchange: Cu²⁺ gives deep blue [Cu(NH3)4(H2O)2]2+\mathrm{[Cu(NH_3)_4(H_2O)_2]^{2+}}; Cr³⁺ gives purple [Cr(NH3)6]3+\mathrm{[Cr(NH_3)_6]^{3+}}. With carbonate, 2+ ions give MCO₃ precipitates; 3+ ions are acidic enough to give the hydroxide + CO₂.

Cambridge 9701 Paper 4Edexcel YCH11 WCH15AQA 7405 Paper 1≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Explain why 3+ aqua ions are more acidic.
  • Describe precipitates with NaOH and NH₃ and their colours.
  • Describe reactions with excess reagents and with carbonate.
  • Identify ions from observations.
Pause and think

Why does Fe³⁺(aq) fizz with sodium carbonate, while Fe²⁺(aq) just gives a precipitate?

Section 2 of 14

Core idea

Start hereTest tubes of colour

Drip sodium hydroxide into metal ion solutions and you get a rainbow of precipitates. Each colour is a fingerprint.

The big idea

Aqua ions lose H⁺ from water ligands. Lose enough to become neutral and a precipitate forms; 3+ ions lose H⁺ more readily, so they're more acidic.

1. Deprotonation

From aqua ion to precipitate (Fe³⁺)
  1. [Fe(H2O)6]3+\mathrm{[Fe(H_2O)_6]^{3+}}
    Hexaaqua ion.
  2. [Fe(H2O)5(OH)]2+\mathrm{[Fe(H_2O)_5(OH)]^{2+}}
    One H⁺ removed by OH⁻.
  3. [Fe(H2O)3(OH)3]\mathrm{[Fe(H_2O)_3(OH)_3]}
    Three removed: neutral, insoluble, orange-brown.
Identifying ions with NaOH and NH₃

Watch the colour and whether excess dissolves it.

Worked exampleCarbonate test

Na₂CO₃ is added to Al³⁺(aq) and to Cu²⁺(aq). Predict observations.

Goal
Observations.
Strategy
Compare acidity of 3+ and 2+ ions.
  1. Al3+: white ppt + fizzing\mathrm{Al^{3+}}:\ \text{white ppt + fizzing}
    Acidic enough to release CO₂; Al(OH)₃ forms.
  2. Cu2+: blue-green ppt\mathrm{Cu^{2+}}:\ \text{blue-green ppt}
    CuCO₃ precipitates.
AnswerAl³⁺ effervesces; Cu²⁺ does not.
✓ Check3+ ions more acidic ✓.

Summary

  • 3+ aqua ions more acidic.
  • Neutral complex = precipitate.
  • Al, Cr amphoteric.
  • Excess NH₃ ligand exchange.
Key words
Amphoteric
Reacts with both acids and bases.
Deprotonation
Loss of H⁺.
HistoryPrussian blue. Prussian blue, discovered around 1706, forms when Fe³⁺ reacts with hexacyanoferrate(II). It became the first modern synthetic pigment and is now a medicine for thallium poisoning.
Fun factRust is a precipitate. The orange-brown of rust is closely related to the Fe(OH)₃ precipitate you see in test tubes.
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