Flame Tests
Difficulty: Easy | Time: 15 minutes | Visual Impact: Very High
Historical Context
Flame tests are among the oldest analytical techniques in chemistry. Alchemists noted that different substances produced different flame colors, though they couldn’t explain why. The systematic study began in the 1750s when Andreas Marggraf used flame color to distinguish sodium and potassium compounds.
The breakthrough came with Robert Bunsen and Gustav Kirchhoff in 1859-1860. Using a spectroscope, they showed that each element produces unique spectral lines - essentially a “fingerprint.” This work led to the discovery of cesium (sky-blue lines) and rubidium (deep red lines), and laid the foundation for atomic spectroscopy.
Today we understand that flame colors arise from electronic transitions. Heat excites electrons to higher energy levels; when they fall back, they emit photons of specific wavelengths characteristic of that element.
Materials
- Metal salt solutions (1g salt in 10mL water each):
- Sodium chloride (Na): yellow
- Potassium chloride (K): lilac/violet
- Potassium sulfate (K): lilac/violet — same as KCl!
- Cupric chloride (Cu): blue-green (intense!)
- Calcium chloride (Ca): orange-red
- Lithium chloride (Li): crimson red
- Cobalt chloride (Co): silvery-white (TOXIC)
- Wire loop (nichrome or platinum) - 5cm wire with 3mm loop
- Bunsen burner or alcohol lamp
- Dilute hydrochloric acid - 10mL (5–10% solution)
- Small containers for each salt solution
Procedure
- Dip wire loop in HCl, then heat in flame until no color (10-15 seconds)
- Dip clean loop in salt solution
- Hold in hottest part of flame (just above blue cone)
- Observe characteristic color for 2-3 seconds
- Clean loop in HCl between each test
Flame Colors
| Metal Ion | Color | Notes |
|---|---|---|
| Sodium (Na⁺) | Intense yellow | Masks other colors |
| Potassium (K⁺) | Lilac/violet | View through blue glass to filter sodium |
| Copper (Cu²⁺) | Blue-green | Very intense |
| Calcium (Ca²⁺) | Orange-red | Brick red |
| Lithium (Li⁺) | Crimson red | Pinkish-red |
| Barium (Ba²⁺) | Yellow-green | Apple green |
| Strontium (Sr²⁺) | Crimson red | Deeper than lithium |
The Science
Heat excites electrons to higher energy levels. When they fall back, they emit specific wavelengths of light unique to each element:
\[E = h\nu = \frac{hc}{\lambda}\]
Each element has different energy level spacings, producing different photon energies (colors). This is the basis of atomic emission spectroscopy and how we know the composition of distant stars.
Crucially, the flame color depends only on the metal ion - not on the anion. Potassium chloride and potassium sulfate produce identical lilac flames. Sodium chloride and sodium carbonate produce identical yellow flames. The anion is irrelevant because it doesn’t survive intact in the high-temperature flame. Testing both KCl and K₂SO₄ back-to-back is a direct, visible demonstration of this principle.
Explore Further
Test the anion: Perform the flame test on KCl and K₂SO₄ side by side. Are the colors identical? This confirms that the color comes from K⁺, not from the chloride or sulfate. Try the same with two sodium salts.
Unknown identification: Have someone prepare three unlabeled solutions from the salt list. Can you identify the metal ion in each based on flame color alone? This is how 19th-century chemists identified elements in rock samples and distant stars.
Blue glass filter: Potassium’s lilac flame is easily masked by trace sodium contamination (sodium gives an overwhelming yellow). View the potassium flame through blue cobalt glass - it filters the yellow and makes the lilac visible. Compare viewing with and without the filter.