Sodium Hydroxide TestOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is the sodium hydroxide test used for?
Sodium hydroxide solution can be added dropwise to a solution of an unknown metal compound to identify which metal ion is present. Several common metal ions form a precipitate with sodium hydroxide, and the colour of that precipitate identifies the metal ion.
Section 2
Which metal ions form white precipitates?
Aluminium, calcium and magnesium ions all form a white precipitate with sodium hydroxide solution. Since all three give the same colour, an extra step is needed to tell them apart: only the aluminium hydroxide precipitate dissolves when an excess of sodium hydroxide solution is added; calcium hydroxide and magnesium hydroxide remain as insoluble white precipitates even in excess.
When asked to distinguish aluminium, calcium and magnesium ions, always mention the excess sodium hydroxide step - the colour test alone cannot tell them apart.
Section 3
Which metal ions form coloured precipitates?
Some transition metal ions give distinctive coloured precipitates with sodium hydroxide solution, which makes them easy to identify without any further steps:
| Metal ion | Precipitate colour |
|---|---|
| Copper(II) | Blue |
| Iron(II) | Green |
| Iron(III) | Brown |
Section 4
How is the test carried out and interpreted?
- Add a small volume of sodium hydroxide solution dropwise to a solution of the unknown compound.
- Observe the colour of any precipitate formed.
- If a white precipitate forms, add more sodium hydroxide solution (excess) and observe whether it redissolves.
- Use the table of results to identify the metal ion present.
A student who stops after seeing a white precipitate and concludes 'magnesium' without testing with excess sodium hydroxide cannot actually rule out aluminium or calcium - the extra step is essential.
Must Know
- Sodium hydroxide solution is added to a metal ion solution to identify the metal ion from the precipitate formed.
- Aluminium, calcium and magnesium ions all give a white precipitate.
- Only aluminium hydroxide dissolves in excess sodium hydroxide solution; calcium and magnesium hydroxide stay insoluble.
- Copper(II) ions give a blue precipitate.
- Iron(II) ions give a green precipitate.
- Iron(III) ions give a brown precipitate.
That's the notes covered.
Carry on to the next subtopic.