Flame Tests and Identification of Metal Ions — AQA GCSE Chemistry (Separate)
Metal ions can be identified by flame tests and by reactions with sodium hydroxide solution.
Flame tests
Some metal ions produce a characteristic flame colour when heated. Dip a clean wire loop in the sample and hold it in a blue Bunsen flame:
| Metal ion | Flame colour |
|---|---|
| Lithium (Li⁺) | crimson (red) |
| Sodium (Na⁺) | yellow |
| Potassium (K⁺) | lilac (purple) |
| Calcium (Ca²⁺) | orange-red |
| Copper (Cu²⁺) | green |
A flame test only works if one metal ion is present (a mixture's colours can mask each other).
Metal hydroxide precipitates
Adding sodium hydroxide solution to a solution of metal ions can form a coloured precipitate:
| Metal ion | Precipitate colour |
|---|---|
| Calcium (Ca²⁺) | white |
| Magnesium (Mg²⁺) | white |
| Aluminium (Al³⁺) | white, but dissolves in excess NaOH |
| Copper(II) (Cu²⁺) | blue |
| Iron(II) (Fe²⁺) | green |
| Iron(III) (Fe³⁺) | brown |
Calcium, magnesium and aluminium all give white precipitates, but aluminium's redissolves in excess sodium hydroxide, distinguishing it.
Exam tips
- Learn the flame colours (Li crimson, Na yellow, K lilac, Ca orange-red, Cu green).
- Learn the hydroxide precipitate colours (Cu blue, Fe²⁺ green, Fe³⁺ brown).
- Aluminium's white precipitate dissolves in excess NaOH.
- Flame tests need a single metal ion present.