Flame tests are a quick analytical method for identifying metal ions by the characteristic color they produce when heated in a flame. When atoms absorb thermal energy, their electrons jump to higher energy levels. As these excited electrons drop back to their ground state, they emit light at specific wavelengths, producing the distinctive colors we observe.
Common Flame Test Colors
The following metal ions produce characteristic colors when introduced into a Bunsen burner flame:
How Flame Tests Work
Flame tests rely on the quantized energy levels of electrons in atoms. When an atom is heated, its electrons absorb energy and transition to higher-energy orbitals (excited states). These excited states are unstable, so electrons quickly fall back to lower energy levels, releasing the absorbed energy as photons of light. The wavelength, and therefore the color, of the emitted light is determined by the energy difference between the two levels, which is unique to each element. This is the same principle behind the Bohr model of the atom and the basis of emission spectroscopy.
Tips for Accurate Flame Tests
To obtain reliable results: clean a nichrome or platinum wire loop by dipping it in concentrated hydrochloric acid (HCl) and holding it in the hottest part of the Bunsen flame until no color is produced. Then dip the clean wire into the sample and return it to the flame. Sodium contamination is the most common problem. Even trace amounts produce an intense yellow that masks other colors. Viewing the flame through cobalt blue glass absorbs the sodium yellow, making it easier to identify potassium's lilac or other subtle colors beneath.
Elements Named After a Color
Long before spectroscopy, chemists named elements for colors they could see with the naked eye, whether the element itself, its vapor, or its compounds:
Named by Humphry Davy for the yellow-green color of chlorine gas.
Named for the deep violet vapor iodine gives off when it sublimes.
The paintbox element: yellow chromates, orange dichromates, green chromium(III) oxide, and the chromophore behind both ruby red and emerald green.
Named for the rose-red color of its salt solutions.
Named for the striking range of colors shown by its salts.
Named for the green color of its salts; it was split from the old element didymium, the twin.
Named by Their Spectral Fingerprint
The flame colors above are more than a lab trick. In the 1860s they became a discovery tool: Robert Bunsen and Gustav Kirchhoff developed spectroscopic analysis into a practical method for identifying elements, and several new elements were then found by the characteristic colored lines they cast through a spectroscope, each named for that color. Two of them, cesium and rubidium, are in the flame-test list above.
The first element ever discovered by spectroscopy (1860), named for its two bright blue spectral lines.
Discovered in 1861 and named for its dark-red spectral lines.
Discovered in 1861 and named for its brilliant green spectral line.
Discovered in 1863 and named for its indigo-blue spectral line.
A Color That Doesn't Count: Bromine
Bromine is a deep reddish-brown liquid, so it is often assumed to be a color name. It is not. Bromine comes from the Greek bromos, meaning stench, for its sharp, choking smell. It is the element people most often miscount as a color name, a reminder that an element's name records whatever struck its discoverer first, sometimes the eyes and sometimes the nose.