Reactions of ions in aqueous solution (A-level only) revision notes

Reactions of Aqueous Metal Ions

Reactions with Sodium Hydroxide

• When metal ions are added to sodium hydroxide (NaOH), they can form metal hydroxides, which may precipitate out of solution. For example:
• Copper(II) ions (Cu²⁺) form a blue precipitate of copper(II) hydroxide (Cu(OH)₂).
• Iron(II) ions (Fe²⁺) produce a green precipitate of iron(II) hydroxide (Fe(OH)₂).
• Iron(III) ions (Fe³⁺) yield a brown precipitate of iron(III) hydroxide (Fe(OH)₃).

Reactions with Ammonia

• Aqueous metal ions can also react with ammonia (NH₃) to form complex ions or precipitates:
• Copper(II) ions (Cu²⁺) react with ammonia to form a deep blue solution of tetraamminecopper(II) complex.
• Zinc ions (Zn²⁺) form a soluble complex with ammonia, resulting in a colorless solution.

Identifying Anions

• Carbonate ions (CO₃²⁻) can be identified by adding dilute acid, which produces carbon dioxide gas (bubbles).
• Sulfate ions (SO₄²⁻) can be detected using barium chloride (BaCl₂), which forms a white precipitate of barium sulfate (BaSO₄).
• Halide ions (Cl⁻, Br⁻, I⁻) can be tested with silver nitrate (AgNO₃), producing white (chloride), cream (bromide), or yellow (iodide) precipitates.

Linking Observations to Conclusions

• It is essential to connect the visual changes observed during these tests to the chemical species present. For instance, the formation of a precipitate indicates the presence of a specific ion.

Practical Skills

• Conducting test-tube reactions to identify cations and anions is a key practical skill. Ensure to follow safety protocols and accurately record observations.