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Dip a clean wire loop into a salt, hold it in a hot flame, and the flame suddenly turns a vivid colour — crimson, lilac, apple green. Each metal gives its own characteristic colour, which makes the flame test one of the oldest and simplest ways to identify an element. It’s also exactly how fireworks get their colours.
The colours
| Metal ion | Flame colour |
|---|---|
| Lithium (Li⁺) | Crimson red |
| Sodium (Na⁺) | Intense yellow-orange |
| Potassium (K⁺) | Lilac (pale purple) |
| Rubidium (Rb⁺) | Red-violet |
| Cesium (Cs⁺) | Blue-violet |
| Calcium (Ca²⁺) | Orange-red (brick red) |
| Strontium (Sr²⁺) | Crimson / bright red |
| Barium (Ba²⁺) | Pale apple green |
| Copper (Cu²⁺) | Blue-green |
| Boron compounds | Bright green |
Colour descriptions vary a little between sources, and the exact shade depends on the flame and the compound.
Why each metal has its own colour
It comes down to electrons and energy levels.
- The heat of the flame gives energy to the metal atoms or ions, exciting some of their electrons to higher energy levels.
- Excited electrons don’t stay there. They fall back down to lower levels, releasing the extra energy as light.
- The energy levels in each element are fixed and unique, so the energy released — and therefore the wavelength, or colour, of the light — is characteristic of that element.
This is the same idea behind Bohr’s model of the atom. Each element has a unique set of emitted wavelengths, called its emission spectrum — a kind of fingerprint.
How to do a flame test
- Clean a nichrome or platinum wire loop by dipping it in dilute hydrochloric acid and heating it until it gives no colour.
- Dip the loop in the acid again and then into the solid sample.
- Hold it at the edge of a hot, blue Bunsen burner flame and observe.
- Clean the loop again before testing the next sample.
(A splint soaked in the solution, or spraying a solution into the flame, also works.)
Limitations
- Sodium overwhelms everything. Even tiny traces of sodium — from your fingers or impure chemicals — give a strong yellow that can hide other colours.
- Potassium is easily masked. Its faint lilac is usually observed through cobalt blue glass, which filters out sodium’s yellow.
- Some colours are similar. Lithium and strontium are both red; experienced observers or a spectroscope can tell them apart.
- Many metals give no useful colour, including magnesium and most transition metals besides copper.
- It identifies elements, not compounds: sodium chloride and sodium sulfate both give sodium yellow.
For precise work, chemists use flame emission spectroscopy or atomic absorption spectroscopy, which measure the exact wavelengths and can detect tiny amounts.
Spectroscopy discovered new elements
In 1859–1860, Robert Bunsen (of the burner) and Gustav Kirchhoff built a spectroscope that split flame colours into sharp lines. Analysing mineral water, they found lines no known element produced, and discovered cesium (named for its sky-blue lines) and then rubidium (deep red lines). Thallium, indium and helium were discovered the same way — helium in the spectrum of the Sun, before anyone found it on Earth. See how elements get their names.
Fireworks
Firework makers use exactly these colours:
- Red: strontium salts (lithium salts are sometimes used too)
- Orange: calcium salts
- Yellow: sodium salts
- Green: barium salts
- Blue: copper compounds — the hardest colour to make bright and pure
- Purple: a mix of strontium (red) and copper (blue)
- White and silver: burning magnesium, aluminium or titanium
The same physics explains the orange glow of old sodium-vapour street lights and the colours of neon signs.
Quick answers
What colour does sodium give in a flame test? An intense yellow-orange.
Why does potassium look lilac? Its main visible emission lines are in the violet and red; together they appear pale purple.
Why do flame tests work? Excited electrons fall back to lower energy levels and emit light of wavelengths unique to each element.
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