Sodium hydroxide solution precipitates metal hydroxides
Definition
Precipitate
An insoluble solid that forms when two solutions are mixed.
- Sodium hydroxide solution is added to a solution of the unknown salt.
- The hydroxide ions react with the metal ion to form an insoluble hydroxide.
- That insoluble solid appears as a precipitate.
- The colour of the precipitate is what identifies the metal ion.
- Adding sodium hydroxide in excess gives a second piece of evidence.
Key Idea
Two observations are available: the colour of the precipitate, and whether it dissolves in excess.
The coloured precipitates
- Copper, Cu2+\text{Cu}^{2+}Cu2+, gives a blue precipitate.
- Iron(II), Fe2+\text{Fe}^{2+}Fe2+, gives a green precipitate.
- Iron(III), Fe3+\text{Fe}^{3+}Fe3+, gives a brown precipitate.
- The reaction for copper is: 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)
- None of these three dissolves when more sodium hydroxide is added.
Example
- Blue: copper. Green: iron(II). Brown: iron(III).
- Iron's two ions are told apart by colour alone, which is why the charge matters.
The two white precipitates are told apart by excess
- Aluminium, Al3+\text{Al}^{3+}Al3+, gives a white precipitate.
- Calcium, Ca2+\text{Ca}^{2+}Ca2+, also gives a white precipitate.
- Adding excess sodium hydroxide separates them.
- The aluminium precipitate dissolves in excess, giving a colourless solution.
- The calcium precipitate does not dissolve, however much is added.
Common Mistake
Stopping at the first white precipitate leaves the two ions indistinguishable.
The ammonium ion gives off ammonia
- Sodium hydroxide solution is added to the sample and the mixture is warmed gently.
- The ammonium ion, NH4+\text{NH}_4^{+}NH4+, releases ammonia gas.
- The reaction is: NH4++OH−→NH3+H2O\text{NH}_4^{+} + \text{OH}^{-} \rightarrow \text{NH}_3 + \text{H}_2\text{O}NH4++OH−→NH3+H2O
- No precipitate forms, since the ammonium ion is not a metal ion.
- The gas is then identified by the test for ammonia.
Note
Warming is needed to drive the gas off, so a cold mixture may give no result.
The chemical test for ammonia
- Damp red litmus paper is held at the mouth of the tube.
- Ammonia is alkaline in solution, so it turns the paper blue.
- The paper must be damp, because the gas has to dissolve before it can act.
- Ammonia also has a sharp, choking smell, though smell alone is not the test.
- A blue result confirms the ammonium ion was present in the original sample.
Self review
- What two observations does a sodium hydroxide test give?
- Give the precipitate colours for copper, iron(II) and iron(III).
- How are aluminium and calcium told apart?
- Why does the ammonium ion give no precipitate?
- Describe the chemical test for ammonia.
