Identifying metal ions
Flame tests
Some give a coloured flame when heated:
| Ion | Flame colour |
|---|---|
| Lithium, Li⁺ | Crimson |
| Sodium, Na⁺ | Yellow |
| Potassium, K⁺ | Lilac |
| Calcium, Ca²⁺ | Orange-red |
| Copper, Cu²⁺ | Green |
Method: clean a nichrome wire loop by dipping it in acid and holding it in a flame until there is no colour. Dip it in the sample, then hold it in the edge of a blue Bunsen flame and note the colour.
If a sample has a of metal ions, the colours can mask each other. Sodium's strong yellow hides other colours.
Key terms
Sodium hydroxide test
Add a few drops of sodium hydroxide solution to a solution of the sample. Many metal ions form a — an insoluble solid — of the metal hydroxide:
| Ion | Precipitate |
|---|---|
| Aluminium, Al³⁺ | White — dissolves when NaOH is added |
| Calcium, Ca²⁺ | White |
| Magnesium, Mg²⁺ | White |
| Copper(II), Cu²⁺ | Blue |
| Iron(II), Fe²⁺ | Green |
| Iron(III), Fe³⁺ | Brown |
To tell the white ones apart: only aluminium hydroxide dissolves in excess NaOH. Calcium gives an orange-red flame test and magnesium gives no flame colour.
For example: CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄
Key terms
Ionic equations for the precipitates (Higher tier only)
Only the metal ion and the hydroxide ions take part in forming the precipitate. The other ions are and are left out:
- Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)
- Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)
- Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)
The number of OH⁻ ions matches the charge on the metal ion, so the hydroxide has no overall charge.