Section 1 of 3Hydroxide Precipitates
Adding sodium hydroxide solution to a solution of metal ions often forms a solid precipitate.
Precipitate
- Precipitate A precipitate is an insoluble solid that forms when two solutions are mixed.
Copper(II)
- Copper(II) Copper(II) ions form a pale blue precipitate of copper(II) hydroxide with sodium hydroxide solution.
Iron(II)
- Iron(II) Iron(II) ions form a green precipitate of iron(II) hydroxide, which darkens when it is exposed to air.
Iron(III)
- Iron(III) Iron(III) ions form a rust-brown precipitate of iron(III) hydroxide, which does not dissolve in excess alkali.
Aluminium
- Aluminium Aluminium ions form a white precipitate that dissolves in excess sodium hydroxide to give a colourless solution.
Magnesium
- Magnesium Magnesium ions form a white precipitate that does not dissolve in excess sodium hydroxide.
Section 2 of 3Ionic Equations
Ionic equations show only the ions that take part in the precipitation.
Ionic Equations
Copper(II)
- Copper(II) Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s), giving a pale blue solid.
Iron(III)
- Iron(III) Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s), giving a rust-brown solid.
Aluminium
- Aluminium Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s), and this solid dissolves in excess sodium hydroxide.
Spectator ions
- Spectator ions Sodium and nitrate ions do not change during the reaction, so they are left out of the ionic equation.
Section 3 of 3Exam Checklist & Tips
8.3.2 – Key Points for the Exam
- Precipitation A reaction in which an insoluble solid forms when two solutions are mixed.
- Hydroxide A compound containing the OH⁻ ion, such as Cu(OH)₂.
- Ionic equation An equation that shows only the ions that react to form a product.
- Spectator ion An ion that is present in solution but does not take part in the reaction.
- Insoluble Does not dissolve in a solvent such as water.
Common Mistakes
- Saying the precipitate is a gas or that it dissolves in water;
- the metal hydroxides formed in these tests are insoluble solids.
- Confusing iron(II) with iron(III) because both are described as iron;
- their precipitates have different colours.
- Assuming all white precipitates are the same metal;
- magnesium and aluminium both give white precipitates, so the excess test is needed.
- Writing the formula of the hydroxide without balancing the number of hydroxide ions in the ionic equation.