📘 CodingMarble Learn

Metal-Aqua Ions: Reactions in Aqueous Solution

In water, a metal ion is held by six water ligands: [M(H₂O)₆]²⁺ or [M(H₂O)₆]³⁺. A 3+ ion pulls hard on the O–H bonds, so it releases H⁺ and its solution is more acidic. Adding OH⁻, NH₃ or CO₃²⁻ removes H⁺ step by step until a neutral hydroxide precipitates. Al(OH)₃ dissolves again in excess OH⁻ (amphoteric); Cu(OH)₂ dissolves in excess ammonia by ligand substitution. 2+ ions give carbonates with CO₃²⁻, but 3+ ions give hydroxides and CO₂ gas.

🎬 Step-by-step story

  1. A metal ion sits in water. Six water molecules join it, each by a lone pair on oxygen. They form an octahedron: [Cu(H₂O)₆]²⁺. The solution is blue.
  2. Now a 3+ ion: [Fe(H₂O)₆]³⁺. Its strong pull weakens one O–H bond. An H⁺ leaves and joins water to make H₃O⁺. That is why 3+ solutions are acidic (pH about 2–3).
  3. Add a little sodium hydroxide. OH⁻ takes away H⁺ until the complex has no charge: [Cu(H₂O)₄(OH)₂]. A neutral complex does not dissolve. A pale blue precipitate falls.
  4. Aluminium is different. Its white precipitate [Al(H₂O)₃(OH)₃] dissolves in excess OH⁻ to make [Al(OH)₄(H₂O)₂]⁻. Reacting with acid and with base: amphoteric.
  5. Excess ammonia is a ligand too. With copper, four NH₃ replace four waters: [Cu(NH₃)₄(H₂O)₂]²⁺, a deep blue solution. With sodium carbonate, a 3+ ion gives a hydroxide and CO₂ bubbles.
  6. Your turn. Pick a metal ion and a reagent. Predict the colour and the formula, then check.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why is the shape octahedral?

Six water ligands spread out as far apart as possible around the metal: up, down, left, right, front, back. Step 1 shows the six positions.

How can a solution of a metal salt be acidic if no acid was added?

The metal ion pulls on the water ligands so strongly that an H⁺ leaves and makes H₃O⁺. Watch the H⁺ drift away in step 2.

Why does the precipitate form only when the complex is neutral?

Charged ions are surrounded and held by water molecules, so they dissolve. A neutral complex has no charge to attract water, so it comes out as a solid. Step 3.

Why does only Al(OH)₃ dissolve in excess NaOH?

Al(OH)₃ can lose one more H⁺ to form the soluble [Al(OH)₄(H₂O)₂]⁻ ion; the iron and copper hydroxides do not. Step 4.

Is ammonia a base or a ligand here?

Both. A little NH₃ acts as a base (precipitate). Excess NH₃ swaps in as a ligand with copper. Step 5.

Why do 2+ and 3+ ions behave differently with carbonate?

3+ ions are acidic enough to make CO₂ from carbonate; 2+ ions are not, so they just form MCO₃. Try both in free play.

Metal-aqua ions and their acidity

When a salt such as CuSO₄ or FeCl₃ dissolves, each metal ion is surrounded by six water molecules. Each water gives a lone pair from O to the metal: a co-ordinate (dative) bond. The result is an octahedral metal-aqua ion, [M(H₂O)₆]ⁿ⁺.

The positive metal ion pulls electron density from the O–H bonds of its water ligands. This weakens them, so an H⁺ can leave (hydrolysis):

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

A 3+ ion has a higher charge density (more charge, smaller size) than a 2+ ion. It polarises the water more, so more H⁺ is released. Solutions of 3+ aqua ions have pH about 2–3; solutions of 2+ ions have pH about 5–6.

Hydroxide precipitates and amphoteric behaviour

A base removes H⁺ from the water ligands one at a time. When the complex has no charge, it is no longer attracted to water and it precipitates as a metal hydroxide:

Amphoteric: aluminium hydroxide reacts with acid and with excess base:

The other hydroxides here do not dissolve in excess NaOH.

Reactions with ammonia and carbonate

Ammonia is a weak base and a ligand. A little NH₃ acts as a base and gives the same hydroxide precipitates as NaOH. In excess, NH₃ can replace water ligands (ligand substitution):

Carbonate ions:

Summary table and Try it

Try it: in free play, choose each ion with "excess NH₃" and then "Na₂CO₃". Say your prediction aloud first. Then make a 4 × 4 table in your notebook and fill it from memory.

Key formulas and definitions

Worked examples

1. Explain why a solution of [Fe(H₂O)₆]³⁺ is more acidic than [Fe(H₂O)₆]²⁺.

Step 1: Fe³⁺ has a larger charge and smaller radius than Fe²⁺: higher charge density. Step 2: It attracts electrons from the O–H bonds of its water ligands more strongly, weakening them. Step 3: So H⁺ is released more easily: [Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅(OH)]²⁺ + H₃O⁺. Answer: more H₃O⁺, lower pH.

2. Write an equation for the reaction of [Cu(H₂O)₆]²⁺ with a little NaOH and give the observation.

Step 1: Two OH⁻ remove two H⁺ to make a neutral complex. Step 2: [Cu(H₂O)₆]²⁺ + 2OH⁻ → [Cu(H₂O)₄(OH)₂] + 2H₂O. Answer: blue solution gives a pale blue precipitate.

3. A colourless solution gives a white precipitate with NaOH that dissolves in excess. Identify the ion and write the equation for dissolving.

Step 1: Colourless + white precipitate suggests Al³⁺. Step 2: Dissolving in excess OH⁻ shows amphoteric Al(OH)₃. Step 3: [Al(H₂O)₃(OH)₃] + OH⁻ → [Al(H₂O)₂(OH)₄]⁻ + H₂O. Answer: Al³⁺.

4. What do you see when excess ammonia is added to [Cu(H₂O)₆]²⁺? Write the overall equation.

Step 1: First a pale blue precipitate of [Cu(H₂O)₄(OH)₂] forms. Step 2: In excess, NH₃ replaces water ligands: [Cu(H₂O)₄(OH)₂] + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2H₂O + 2OH⁻. Answer: precipitate dissolves to give a deep blue solution.

5. Sodium carbonate is added separately to Fe²⁺(aq) and Fe³⁺(aq). Explain the different results.

Step 1: Fe²⁺ is weakly acidic, so the carbonate ion just combines with it: FeCO₃, a green precipitate, no gas. Step 2: Fe³⁺ is acidic enough to give H⁺ to CO₃²⁻, making CO₂ gas. Step 3: 2[Fe(H₂O)₆]³⁺ + 3CO₃²⁻ → 2[Fe(H₂O)₃(OH)₃] + 3CO₂ + 3H₂O. Answer: Fe²⁺: green FeCO₃; Fe³⁺: brown precipitate and bubbles.

6. How many moles of CO₂ form when 0.020 mol of [Al(H₂O)₆]³⁺ reacts fully with carbonate?

Step 1: 2[Al(H₂O)₆]³⁺ + 3CO₃²⁻ → 2[Al(H₂O)₃(OH)₃] + 3CO₂ + 3H₂O. Step 2: Ratio Al : CO₂ = 2 : 3. Step 3: 0.020 × 3/2 = 0.030 mol. Answer: 0.030 mol CO₂.

Common mistakes

Practice quiz

1. The shape of [Fe(H₂O)₆]³⁺ is:
2. Which ion's aqua solution is most acidic?
3. Colour of the precipitate from Fe³⁺(aq) and NaOH:
4. Which hydroxide dissolves in excess NaOH?
5. Excess ammonia with Cu²⁺(aq) gives:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is a metal-aqua ion?

A metal ion bonded to water molecules by co-ordinate bonds, usually six in an octahedron, such as [Fe(H₂O)₆]³⁺.

Why is Fe³⁺ solution yellow-brown and not violet?

Pure [Fe(H₂O)₆]³⁺ is pale violet, but hydrolysis gives [Fe(H₂O)₅(OH)]²⁺ and similar species, which colour the solution yellow-brown.

What is the test for copper(II) ions?

Add ammonia solution: a pale blue precipitate forms, then dissolves in excess to give a deep blue solution.

Where this is taught

England (GCSE, A level)Year 133.2 Inorganic chemistry

Learn first

Learn next

Related lessons

All Chemistry lessons