2.4.6Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Flame Tests — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 31 of 53 Unit quiz
Part 1 of 4Method
Flame tests identify metal ions through the colours emitted when their compounds are heated.
1
Clean a nichrome or platinum wire loop by dipping it in concentrated hydrochloric acid and heating it in a blue flame.
2
Repeat until the flame shows no colour, which means no earlier sample is contaminating the test.
3
Dip the clean loop into the solid sample, moistened with a little acid if needed, and place it at the edge of the blue flame.
4
Observe the colour of the flame, avoiding the glow of the hot wire, and compare it with known colours.
5
Test only one ion at a time because a mixture of ions will give mixed colours and unclear results.
6
Record the colour straight away, because lilac fades quickly and can be hidden by yellow from traces of sodium.
Part 2 of 4Colours
Lithium
- Lithium ions, Li⁺, give a crimson (red) flame when heated strongly in a Bunsen flame.
Sodium
- Sodium ions, Na⁺, give an intense yellow flame that can mask other colours.
Potassium
- Potassium ions, K⁺, give a lilac flame, which is pale and easily hidden by sodium.
Calcium
- Calcium ions, Ca²⁺, give a brick-red or orange-red flame, while strontium is a clearer red.
Barium and copper
- Barium ions, Ba²⁺, give a pale green flame, while copper(II) ions, Cu²⁺, give blue-green.
| Ion | Flame colour |
|---|---|
| Li⁺ | Crimson |
| Na⁺ | Yellow |
| K⁺ | Lilac |
| Ca²⁺ | Brick red |
| Sr²⁺ | Red |
| Ba²⁺ | Pale green |
| Cu²⁺ | Blue-green |
Part 3 of 4Why Flames Are Coloured
Why Flames Are Coloured
Excitation
- Heat gives energy to an outer electron so that it moves to a higher energy level.
Emission
- The excited electron is unstable and falls back to a lower level, releasing energy as light.
Wavelength
- The energy gap between levels fixes the wavelength of light, so each element has a characteristic colour.
Magnesium
- Magnesium shows no flame colour because the emitted light lies outside the visible region.
Limits of the test
- Flame tests suit metal ions only, and lithium and strontium both look red, so a further test may be needed to tell them apart.
Safety
- Wear eye protection and keep flammable liquids away from the flame, because some metal salts are toxic when heated.
Part 4 of 4Exam Checklist & Tips
2.4.6 – Key Points for the Exam
- Flame test A test that identifies metal ions from the colour they give to a flame.
- Excited state A higher energy level to which an electron is promoted by absorbing energy.
- Emission The release of light energy as electrons fall to lower energy levels.
- Nichrome wire An unreactive alloy wire used to hold samples in a flame.
Common Mistakes
- Saying the colour comes from the metal burning; it is caused by electron transitions.
- Confusing lilac (potassium) with the red of lithium or strontium.
- Not cleaning the wire, so sodium contamination masks other colours.
- Expecting every metal ion to give a colour; magnesium does not.
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