Sodium hydroxide test: metal ions form coloured hydroxide precipitates
Definition
Metal hydroxide
An ionic compound containing positive metal ions and negative hydroxide ions.
Definition
Precipitate
A precipitate is an insoluble solid that forms when substances in solution react.
Definition
Hydroxide ion
A negatively charged group of atoms with the formula OH−\text{OH}^-OH− that is present in metal hydroxides.
- Adding sodium hydroxide solution to a solution of a metal ion can form an insoluble metal hydroxide.
- This solid appears as a coloured precipitate, and its colour identifies the metal ion.
- The metal ion reacts with hydroxide ions, OH−\text{OH}^-OH−, from the sodium hydroxide.
Key Idea
- The colour of the precipitate is the clue to which metal ion is present.
- Three metal ions give a white precipitate, so a further step is needed to tell them apart.
The precipitate colours to learn
- Copper(II) (Cu2+\text{Cu}^{2+}Cu2+): blue precipitate.
- Iron(II) (Fe2+\text{Fe}^{2+}Fe2+): green precipitate.
- Iron(III) (Fe3+\text{Fe}^{3+}Fe3+): brown precipitate.
- Calcium (Ca2+\text{Ca}^{2+}Ca2+), magnesium (Mg2+\text{Mg}^{2+}Mg2+) and aluminium (Al3+\text{Al}^{3+}Al3+): white precipitate.
- Aluminium is the odd one out: its white precipitate dissolves in excess sodium hydroxide, while calcium and magnesium stay white.
Exam technique
- Give the ion and colour together, and add the charge, for example copper(II).
- To separate the three white results, add excess sodium hydroxide: only the aluminium hydroxide redissolves.
Ionic equations for the precipitates (higher tier)
- Copper(II): Cu2+(aq)+2OH−(aq)→Cu(OH)2(s)\text{Cu}^{2+}(aq) + 2\text{OH}^-(aq) \rightarrow \text{Cu(OH)}_2(s)Cu2+(aq)+2OH−(aq)→Cu(OH)2(s).
- Iron(II): Fe2+(aq)+2OH−(aq)→Fe(OH)2(s)\text{Fe}^{2+}(aq) + 2\text{OH}^-(aq) \rightarrow \text{Fe(OH)}_2(s)Fe2+(aq)+2OH−(aq)→Fe(OH)2(s).
- Iron(III): Fe3+(aq)+3OH−(aq)→Fe(OH)3(s)\text{Fe}^{3+}(aq) + 3\text{OH}^-(aq) \rightarrow \text{Fe(OH)}_3(s)Fe3+(aq)+3OH−(aq)→Fe(OH)3(s).
- The number of hydroxide ions matches the charge on the metal ion, and the state symbol (s)(s)(s) shows the solid precipitate.
Self review
- What is seen when sodium hydroxide is added to copper(II), iron(II) and iron(III) ions?
- Which three metal ions give a white precipitate?
- How can you tell aluminium apart from calcium and magnesium?
- Write the ionic equation for the reaction of iron(III) ions with hydroxide ions.
