Metal hydroxide precipitate tests
Big Manny demonstrates how to use dilute sodium hydroxide solution to detect and identify metal ions with a GCSE Chemistry practical experiment
Dilute sodium hydroxide solution is used in tests for some metal Electrically charged particle, formed when an atom or molecule gains or loses electrons., which form metal hydroxides that are Unable to dissolve in a particular solvent. For example, sand is insoluble in water.. This means that the metal hydroxides appear as A suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction.. For example, copper sulfate solution reacts with a few drops of sodium hydroxide solution:
copper sulfate + sodium hydroxide → sodium sulfate + copper hydroxide
CuSO4(aq) + 2NaOH(aq) → Na2SO4(aq) + Cu(OH)2(s)
Copper hydroxide forms a blue precipitate.
The table shows the coloured precipitates formed by five common metal ions.
| Metal ion | Precipitate colour |
| Aluminium, Al3+ | White |
| Calcium, Ca2+ | White |
| Magnesium, Mg2+ | White |
| Copper(II), Cu2+ | Blue |
| Iron(II), Fe2+ | Green |
| Iron(III), Fe3+ | Brown |
| Metal ion | Aluminium, Al3+ |
|---|---|
| Precipitate colour | White |
| Metal ion | Calcium, Ca2+ |
|---|---|
| Precipitate colour | White |
| Metal ion | Magnesium, Mg2+ |
|---|---|
| Precipitate colour | White |
| Metal ion | Copper(II), Cu2+ |
|---|---|
| Precipitate colour | Blue |
| Metal ion | Iron(II), Fe2+ |
|---|---|
| Precipitate colour | Green |
| Metal ion | Iron(III), Fe3+ |
|---|---|
| Precipitate colour | Brown |
Distinguishing between aluminium ions, calcium ions and magnesium ions
A few drops of dilute sodium hydroxide solution react to form a white precipitate with aluminium ions, calcium ions and magnesium ions. However, if excess sodium hydroxide solution is added:
- the aluminium hydroxide precipitate dissolves to form a colourless solution
- the calcium hydroxide precipitate is unchanged
- the magnesium hydroxide solution is unchanged
This means that using sodium hydroxide can give a positive result for aluminium ions, but it cannot distinguish between calcium and magnesium ions.
Question
A green precipitate forms when dilute sodium hydroxide solution is added to a sample in solution. Identify the metal ion present in the original solution.
Iron(II) ions, Fe2+, are present.
The ionic equations
The precipitation reactions can be represented using A chemical equation that shows how positively charged ions join with negatively charged ions to make a compound., which only include the ions which are involved in the formation of the precipitate. They ignore the An ion that is exactly the same on both sides of an ionic equation., which are present but not involved. For example:
Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)
In this equation, we can see that:
- the copper ion has a charge of 2+
- therefore two hydroxide ions are needed to balance this because they each have a charge of -1
- therefore the formula of the hydroxide precipitate has two OH ions in it
In the precipitation reaction which identifies the iron(III) ion as being present, the ionic equation is:
Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)
This is because three hydroxide ions are needed to react with the Fe3+ ion.
The spectator ions which are ignored in these equations are the sodium ion (Na+) from the NaOH, and the An atom or group of atoms that have gained electrons and become negatively charged. Most anions are formed from non-metals. from the metal A substance formed by the chemical union of two or more elements., eg the sulfate ion (SO42-) if the copper compound was copper sulfate.
Question
Write a balanced ionic equation to show the reaction of iron(II) ions with hydroxide ions.
Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)