Metal-aqua ions and their acidity

In water, metal ions do not float about bare — they are surrounded by six water ligands in an octahedral complex:

• [M(H₂O)₆]²⁺ — for Fe and Cu
• [M(H₂O)₆]³⁺ — for Al and Fe

These solutions are acidic. The metal ion polarises the water ligand: it withdraws electron density from the O–H bond, weakening it, so a proton is released to a water molecule in the bulk solution:

The key explanation, in AQA’s own terms: [M(H₂O)₆]³⁺ is much more acidic than [M(H₂O)₆]²⁺ because the 3+ ion has a higher charge and a smaller radius — a higher charge/size ratio. It polarises the O–H bond more strongly, so the proton is released more readily. That is why Fe³⁺(aq) has a pH of about 2 but Fe²⁺(aq) is only weakly acidic.

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Reactions with hydroxide and ammonia

Both OH⁻ and NH₃ act as bases here: they remove protons from the water ligands until the complex is neutral — and a neutral complex is insoluble, so it precipitates.

• 2+ ions lose two protons: [M(H₂O)₆]²⁺ + 2OH⁻ → [M(H₂O)₄(OH)₂] + 2H₂O
• 3+ ions lose three protons: [M(H₂O)₆]³⁺ + 3OH⁻ → [M(H₂O)₃(OH)₃] + 3H₂O

The colours to memorise:

• Fe²⁺: pale green solution → green precipitate (which darkens on standing as it is oxidised in air to brown Fe(III)).
• Fe³⁺: violet or yellow-brown solution → brown precipitate.
• Cu²⁺: blue solution → blue precipitate.
• Al³⁺: colourless solution → white precipitate.

The two "excess" tests that separate everything. In excess NaOH, only the amphoteric aluminium hydroxide redissolves: [Al(H₂O)₃(OH)₃] + OH⁻ → [Al(H₂O)₂(OH)₄]⁻ + H₂O (a colourless solution). In excess NH₃, only the copper precipitate redissolves — by ligand substitution, not deprotonation — to give the deep blue [Cu(NH₃)₄(H₂O)₂]²⁺.

Which ion gives each observation?

Tap an observation, then tap the aqueous ion responsible.

Reactions with sodium carbonate

This is the reaction that separates 2+ from 3+ ions, and the reason is acidity.

• 2+ aqua ions are only weakly acidic — not acidic enough to release CO₂. The carbonate simply precipitates the metal carbonate: Cu²⁺ + CO₃²⁻ → CuCO₃ (a green-blue precipitate); Fe²⁺ + CO₃²⁻ → FeCO₃ (a green precipitate). No gas is seen.
• 3+ aqua ions are acidic enough to react with the carbonate as an acid, releasing CO₂ gas and precipitating the hydroxide:

The diagnostic: effervescence with sodium carbonate means a 3+ aqua ion. A precipitate with no gas means a 2+ ion. This single test distinguishes Fe³⁺ from Fe²⁺ instantly.

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Match the ion and reagent to the observation

Tap a test on the left, then its observation on the right.

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Deprotonation is not ligand substitution

• Two different reaction types look similar. Forming the hydroxide precipitate is deprotonation — OH⁻ or NH₃ acting as a base, removing H⁺ from a water ligand. Redissolving the copper precipitate in excess ammonia is ligand substitution. Name the right one.
• The acidity explanation is charge/size ratio. "The 3+ ion has a higher charge and smaller radius, so it polarises the O–H bond of the water ligand more, releasing H⁺ more readily."
• Effervescence with carbonate means a 3+ ion. A 2+ ion just gives the metal carbonate precipitate — no gas.
• Only Al(OH)₃ redissolves in excess NaOH (it is amphoteric); only Cu²⁺ redissolves in excess NH₃.

Writing the precipitate formula: the neutral complex, e.g. [Fe(H₂O)₃(OH)₃] for a 3+ ion (three protons removed) or [Cu(H₂O)₄(OH)₂] for a 2+ ion (two removed). The complex must end up with zero overall charge — that is why it is insoluble.

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The big ideas to know

That is the whole of AQA 3.2.6. Press Finish to see your score.