Flame tests
- Dip a clean nichrome wire loop into the sample and hold it in a blue Bunsen flame.
- Lithium: crimson. Sodium: yellow. Potassium: lilac. Calcium: orange-red. Copper: green.
- If a sample contains a mixture of ions, some flame colours can be hidden (masked).
Metal hydroxides
- Add a few drops of sodium hydroxide solution to a solution of the compound.
- Aluminium, calcium and magnesium ions form white precipitates. Only aluminium hydroxide dissolves when excess sodium hydroxide is added.
- Copper(II) forms a blue precipitate, iron(II) a green precipitate and iron(III) a brown precipitate.
Carbonates, halides and sulfates
- Carbonates: add dilute acid; carbon dioxide is produced, which turns limewater milky.
- Halides: add dilute nitric acid, then silver nitrate solution. Chloride gives a white precipitate, bromide a cream precipitate and iodide a yellow precipitate.
- Sulfates: add dilute hydrochloric acid, then barium chloride solution. A white precipitate forms.
- The acid is added first to remove carbonate ions, which would also form precipitates.
Instrumental methods
- Instrumental methods are accurate, sensitive and rapid.
- In flame emission spectroscopy, a sample is put into a flame and the light given out is passed through a spectroscope. The line spectrum identifies the metal ions and shows their concentration.
Key terms
- Flame test
- Holding a sample in a flame to identify metal ions by the flame colour.
- Precipitate
- An insoluble solid that forms when two solutions are mixed.
- Halide ion
- A negative ion from a halogen, such as chloride, bromide or iodide.
- Silver nitrate test
- The test for halide ions, using nitric acid and silver nitrate solution.
- Barium chloride test
- The test for sulfate ions, which forms a white precipitate.
- Flame emission spectroscopy
- An instrumental method that analyses the light given out by ions in a flame.
- Line spectrum
- The pattern of coloured lines produced by a particular metal ion.