Identification of Ions — Flame Tests and PrecipitatesAQA GCSE Chemistry: Revision notes
Section 1
How Do Flame Tests Identify Metal Ions?
A flame test identifies certain metal cations by the colour they produce when heated strongly in a flame:
| Metal ion | Flame colour |
|---|---|
| Lithium | Crimson |
| Sodium | Yellow |
| Potassium | Lilac |
| Calcium | Orange-red |
| Copper | Green |
If a sample is a mixture of ions, some flame colours may be masked by others (for example, a strong sodium yellow can hide a weaker potassium lilac).
A mixture of ions can mask a flame colour — a negative flame test result doesn't always mean an ion is absent.
Section 2
How Does Sodium Hydroxide Identify Metal Ions?
Adding sodium hydroxide solution to a solution of a metal ion can produce a coloured precipitate, helping identify the ion:
| Metal ion | Precipitate with NaOH | Dissolves in excess NaOH? |
|---|---|---|
| Aluminium | White | Yes |
| Calcium | White | No |
| Magnesium | White | No |
| Copper(II) | Blue | No |
| Iron(II) | Green | No |
| Iron(III) | Brown | No |
Aluminium, calcium and magnesium all give a white precipitate, but only aluminium hydroxide dissolves in excess sodium hydroxide, which is how you tell them apart.
White precipitate + dissolves in excess NaOH = aluminium. White precipitate + stays insoluble = calcium or magnesium.
Section 3
How Do You Test for Anions?
Different anions can be identified using specific chemical tests:
- Carbonates: react with dilute acid to produce carbon dioxide gas, identified because it turns limewater milky (cloudy)
- Halide ions: in solution, produce a precipitate with silver nitrate solution (in the presence of dilute nitric acid) — silver chloride is white, silver bromide is cream, silver iodide is yellow
- Sulfate ions: in solution, produce a white precipitate with barium chloride solution (in the presence of dilute hydrochloric acid)
Testing an unknown white solid: dissolve it, add barium chloride and dilute hydrochloric acid — a white precipitate confirms sulfate ions are present.
Section 4
Why Use Instrumental Methods of Analysis?
Instrumental methods of analysis use machines rather than chemical tests. Compared with chemical tests, instrumental methods are:
- Accurate
- Sensitive
- Rapid
One key instrumental method is flame emission spectroscopy, used to analyse metal ions in solution. A sample is put into a flame and the light produced is analysed to identify and measure the concentration of metal ions, often by comparing the result against reference data.
Must Know
- Flame tests: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green — mixtures can mask colours
- Sodium hydroxide: aluminium, calcium and magnesium give white precipitates, but only aluminium's dissolves in excess NaOH; copper gives blue, iron(II) gives green, iron(III) gives brown
- Carbonates + acid → CO2 (turns limewater milky); halides + silver nitrate → precipitate (white/cream/yellow for chloride/bromide/iodide); sulfates + barium chloride → white precipitate
- Instrumental methods (e.g. flame emission spectroscopy) are more accurate, sensitive and rapid than chemical tests
That's the notes covered.
Carry on to the next subtopic.