Testing for aqueous metal ions
Dilute sodium hydroxide solution reacts with some Shiny element that is a good conductor of electricity and heat, and which forms basic oxides. Electrically charged particle, formed when an atom or molecule gains or loses electrons. in Mixture formed by a solute and a solvent., forming metal hydroxides. Some of these metal hydroxides are Unable to dissolve in a particular solvent. For example, sand is insoluble in water., so they 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 |
| Iron(II), Fe2+ | Green |
| Iron(III), Fe3+ | Orange-brown |
| Copper(II), Cu2+ | Blue |
| Calcium, Ca2+ | White |
| Zinc, Zn2+ | White |
| Metal ion | Iron(II), Fe2+ |
|---|---|
| Precipitate colour | Green |
| Metal ion | Iron(III), Fe3+ |
|---|---|
| Precipitate colour | Orange-brown |
| Metal ion | Copper(II), Cu2+ |
|---|---|
| Precipitate colour | Blue |
| Metal ion | Calcium, Ca2+ |
|---|---|
| Precipitate colour | White |
| Metal ion | Zinc, Zn2+ |
|---|---|
| Precipitate colour | White |
Distinguishing between calcium ions and zinc ions
A few drops of dilute sodium hydroxide solution react to form a white precipitate with calcium ions and with zinc ions. However, if excess sodium hydroxide solution is added:
- the calcium hydroxide precipitate is unchanged
- the zinc hydroxide precipitate dissolves to form a colourless solution
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.