Flame Tests

Metal ion identification by flame color

Beginner🕐15 min▲Moderate hazardspectroscopyanalytical

Summary

Different metal ions burn with different flame colours — sodium’s yellow, copper’s blue-green, lithium’s crimson — because heated electrons emit light at each element’s characteristic wavelengths. You’ll be able to explain why the colours are sharp and element-specific, why the anion doesn’t matter, and how this became atomic spectroscopy.

History

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 Bunsen¹ and Gustav Kirchhoff² 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.

Hazards & preparation

Warning

PPE: safety glasses; tie back hair and sleeves; an adult runs the flame.

• Open flame. Keep flammables away and have a way to extinguish it.
• The boron test uses methanol/ethanol, which are flammable — only wet the loop, keep the bottle capped and well away from the flame.
• Cobalt chloride is toxic (gloves; or omit), and dilute hydrochloric acid for cleaning is corrosive.

Disposal: the small salt solutions are dilute — flush with water (hold any cobalt solution for hazmat). See the Safety page.

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!) — or copper sulfate, same result
  • Calcium chloride (Ca): orange-red
  • Lithium chloride (Li): crimson red
  • Cobalt chloride (Co): silvery-white (TOXIC)
  • Boric acid (B): bright green — dissolve 1g in a few mL of methanol or ethanol
• 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

1. Dip the wire loop in HCl, then heat it in the flame until it adds no colour (10-15 seconds).
2. Dip the clean loop in a salt solution.
3. Hold it in the hottest part of the flame (just above the blue cone).
4. Observe the characteristic colour for 2-3 seconds.
5. Clean the loop in HCl between each test.

What you should see

Each salt paints the flame its own colour: an intense, hard-to-miss yellow for sodium; a soft lilac for potassium (easily drowned by any sodium); a vivid blue-green for copper; orange-red (brick) for calcium; a pink-tinged crimson for lithium; and a striking bright green for boron in alcohol. Test KCl and K₂SO₄ back to back — identical lilac.

Symptom	Likely cause	Fix
Everything looks yellow	Sodium contamination on the loop	Clean the loop in HCl and reheat until colourless before each test
Colour too brief to see	Solution too dilute or loop barely wet	Use a more concentrated solution; load the loop well
Potassium’s lilac invisible	Masked by sodium’s yellow	View through blue cobalt glass to filter the yellow
Weak boron green	Not enough alcohol / too little heat	Use boric acid dissolved in methanol/ethanol; hold in the hottest flame

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
Boron (B)	Bright green	Use boric acid in ethanol — one of the most vivid greens
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.

Questions to Explore

1. Why do electrons emit specific colours? Why is each colour tied to a particular element rather than a continuous range?

   Hint / answer

   An atom’s electrons can only occupy fixed energy levels, so when an excited electron drops back it releases a photon of one exact energy — one colour. Because every element has its own unique set of level spacings, each emits its own set of colours: an atomic fingerprint.

2. Why is sodium’s yellow so overwhelming? Trace sodium drowns everything. What does intensity say about the transition?

   Hint / answer

   Sodium’s yellow transition is extremely probable (a strongly “allowed” transition) and easily excited at flame temperatures, so even a few sodium atoms emit a lot of yellow light. That’s why the tiniest contamination swamps fainter colours.

3. Why doesn’t the anion affect the colour? KCl and K₂SO₄ both give lilac. Why don’t chloride or sulfate contribute?

   Hint / answer

   In the flame the salt breaks apart and it’s the metal atoms that get excited and emit visible light. The anions either don’t emit in the visible range or are broken up, so only the metal’s colour shows — the same metal gives the same colour regardless of its partner.

4. How did this discover new elements? Bunsen and Kirchhoff found unfamiliar lines. Why does that mean a new element?

   Hint / answer

   Each element’s line pattern is unique and unchanging, so a set of lines matching no known element must come from a new one. A new compound would still only show the lines of its known constituent elements — new lines mean a new element.

5. How do fireworks engineers use this? Same salts (Li red, Ba green, Sr crimson). What extra challenges do they face?

   Hint / answer

   They must make bright, pure colour at altitude while also burning fuel/oxidizer and launching the shell — sodium impurities (yellow) must be kept out, and the metal salts, binders, and temperature carefully balanced so the intended colour dominates.

Going further

• Test the anion rule. Run KCl and K₂SO₄ (and two sodium salts) side by side to confirm the colour comes from the metal, not the anion.
• Identify an unknown. Have someone hand you unlabeled salt solutions and name the metal from the flame colour alone.
• See the yellow filtered out. View potassium’s lilac through blue cobalt glass to cut sodium contamination — the trick 19th-century chemists used.

Footnotes

1. Robert Bunsen — German chemist (1811–1899) who, with Kirchhoff, developed flame spectroscopy and the Bunsen burner.

2. Gustav Kirchhoff — German physicist (1824–1887) who co-founded spectroscopy with Bunsen.