2.8.2Atoms, Elements and Compounds
Flame Tests — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 19 of 26 Unit quiz
Part 1 of 5Carrying Out a Flame Test
A clean wire is essential, because any residue will mix its colour with the sample.
1
Clean a nichrome or platinum wire loop by dipping it into dilute hydrochloric acid and holding it in a blue Bunsen flame until no colour is seen.
2
Dip the cleaned loop into the solid sample or the solution being tested.
3
Place the loop at the edge of the blue Bunsen flame, where the sample is heated strongly, and observe the colour.
4
Repeat the clean and test steps between samples to avoid mixed colours.
Part 2 of 5Flame Colours of Cations
Each metal ion emits a characteristic colour when heated in a flame.
Lithium
- Lithium Lithium ions give a red flame, which is useful for identifying lithium salts.
Sodium
- Sodium Sodium ions give a strong yellow flame, which often masks other colours.
Potassium
- Potassium Potassium ions give a lilac flame, which is seen best through cobalt glass.
Calcium
- Calcium Calcium ions give an orange-red flame, which is distinct from the red of lithium.
Copper
- Copper Copper(II) ions give a blue-green flame.
| Cation | Flame colour |
|---|---|
| Li⁺ | Red |
| Na⁺ | Yellow |
| K⁺ | Lilac |
| Ca²⁺ | Orange-red |
| Cu²⁺ | Blue-green |
Part 3 of 5Why the Test Works
Why the Test Works
Energy
- Energy Heat excites electrons in the metal ion, which then release light of a particular colour as they fall back.
Contamination
- Contamination Any other ion left on the wire will produce its own colour, which can hide the true result.
Acid first
- Acid first Dipping the wire in acid dissolves residues left from earlier samples, so they cannot add their own colours to the flame.
Limitation
- Limitation Flame tests identify only the cation, and mixtures of metal ions can be difficult to interpret.
Part 4 of 5Worked Example
Question
- A white solid gives a lilac colour in a flame test.
- Identify the metal ion present and explain the result.
- Compare the observed colour with the flame colour table.
- Lilac is the flame colour of potassium ions.
- Potassium is therefore present in the solid, and the test should be repeated on a clean wire to confirm.
Answer The metal ion is potassium, K⁺, because potassium gives a lilac flame.
Part 5 of 5Exam Checklist & Tips
2.8.2 – Key Points for the Exam
- Flame test A test used to identify metal cations by the colour they give to a flame.
- Cation A positively charged ion, such as Na⁺ or Cu²⁺.
- Nichrome An alloy used for wire loops in flame tests because it does not react or colour the flame.
- Blue flame A hot, non-luminous Bunsen flame used for flame tests.
Common Mistakes
- Confusing the colour of a flame test with a precipitate colour; flame tests use the colour of the flame.
- Using a wire that has not been cleaned, which mixes colours and gives a false result.
- Using a yellow Bunsen flame, which is too cool and makes it hard to see the true colour.
- Stating that the flame test identifies an anion; it only identifies metal cations.
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