2.7.2Inorganic Chemistry
Flame Tests — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 36 of 52
Part 1 of 4Purpose of a flame test
What it detects
- What it detects A flame test identifies the positive metal ion (cation) in a compound from the colour it gives a flame.
- Different metal ions give different, characteristic colours.
Why colours appear
- Why colours appear Heating excites electrons in the metal ions;
- as they fall back they give out light of a particular colour.
Limitation
- Limitation Only some metals give clear colours, and a mixture of ions can mask each other, so the test works best on a single pure compound.
Part 2 of 4Method
1
Wear eye protection and set the Bunsen burner to a blue (roaring) flame.
2
Clean a nichrome wire loop by dipping it in dilute hydrochloric acid, then heating it in the flame until it gives no colour.
3
Dip the clean loop into the solid or solution to be tested.
4
Hold the loop at the edge of the blue flame and note the colour.
5
Clean the wire again with acid and flame before testing the next sample.
6
Compare the colour with known results to name the metal ion.
Part 3 of 4Flame colours
| Metal ion | Flame colour |
|---|---|
| Lithium, Li+ | Red (crimson) |
| Sodium, Na+ | Yellow |
| Potassium, K+ | Lilac |
| Calcium, Ca2+ | Orange-red |
| Copper(II), Cu2+ | Blue-green |
| Barium, Ba2+ (extension) | Green |
Part 4 of 4Getting good results
Reliable flame tests
Cleaning the wire
- Cleaning the wire Contamination from an earlier sample would add a second colour, and one colour can hide another.
Flame choice
- Flame choice Use a hot, blue flame; a yellow flame is cooler and gives its own colour.
Recording results
- Recording results Prepare a table listing every sample and its flame colour, then use the table to name the unknown.
Worked example
- Worked example A lilac flame shows potassium, yellow shows sodium, crimson shows lithium, green shows barium and red-orange shows calcium.
Key terms
| Term | Meaning |
|---|---|
| Cation | A positively charged ion, such as a metal ion. |
| Nichrome | A nickel-chromium alloy wire that does not give a flame colour of its own. |
| Contamination | Unwanted material that spoils a test result. |
| Roaring flame | The hot, blue Bunsen flame with the air hole open. |
| Unknown | A sample whose identity is to be found out by testing. |
Avoid these mistakes
1
Not cleaning the wire between samples, so the colours mix and a wrong ion is named.
2
Using a yellow safety flame, which is too cool and masks the colour.
3
Mixing up red lithium and orange-red calcium; check carefully against reference colours.
4
Saying the test identifies the negative ion. A flame test only identifies the metal cation.
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