Why Do We See Colors in a Flame Test?

Why do we see colors in a flame test?

The electrons in an atom occupy different energy levels, as you know. When all of the electrons are at the lowest possible energy level they are said to be in the ground state. Electrons do not always stay in the ground state. Sometimes they can be promoted to a higher-energy electron shell. When an atom is in the flame, an electron in the outer shell of that atom receives energy from the flame and jumps up to a higher-energy shell position. When an electron is in a higher-energy shell it is said to be in an excited state.

Electrons in excited states do not usually stay in them for very long. When electrons lose their energy they do so by emitting a photon of light. Photons are particles with energy but no mass. Their energy is directly proportional to the frequency of the light. The photons emitted precisely match the quantum energy difference between the excited state and the ground state.

Is the flame test fun? Yes!!!! But… what is/was a flame test used for?

The light produced by very hot atoms in the gaseous state is a unique spectrum for each element. To observe the spectrum requires the use of a prism, diffraction grating, or spectroscope. (We will look at line spectrum of various elements using a spectroscope next week.) However, before complex instruments were invented, to observe elemental spectra chemists sometimes identified metals in compounds by doing a flame test.

Further explanation of the flame test:

Salts are a type of compound that include a metal and a non-metal. Sodium chloride (NaCl) is the most familiar example of a salt but others include calcium chloride(CaCl2) and copper(II) chloride (CuCl2). In flame tests, salts that are dissolved in water are evaporated using a hot flame. In the flame, the metal atoms become excited and produce their characteristic spectrum of light. However, since the observer does not use a spectroscope only one color is observed. Many metals produce a unique single color under these conditions, while other metals produce very similar colors that only practiced eye can distinguish. An experienced scientist, based on a comparison between the color observed and the known colors produced by different metallic salts can identify the salt. We will try our ability to identify an unknown salt during the flame test lab.

Real world use:

This ability of metal atoms to produce these colors is put to use in the making of fireworks. By including different metal salts, or mixtures of metal salts, in the exploding shell of a firework, firework manufactures can produce beautiful displays in nearly all the colors of the rainbow.

Optional: If you are interested in learning more about the chemistry of fireworks refer to the below sites. (Use the document posted on our website for active links.

·  NOVA from WGBH: Fireworks! (http://www.pbs.org/wgbh/nova/fireworks/)

·  Chemical of the Week: Fireworks! from the University of Wisconsin (http://www.scifun.org/CHEMWEEK/fireworks/fireworks.htm)

·  A Chemical and Engineering News article about Fireworks (http://pubs.acs.org/cen/whatstuff/stuff/7927sci3.html)

Limitations of the Flame Test

·  The test cannot detect low concentrations of most ions.

·  The brightness of the signal varies from one sample to another. For example, the yellow emission from sodium is much brighter than the red emission from the same amount of lithium.

·  Impurities or contaminants affect the test results. Sodium, in particular, is present in most compounds and will color the flame. Sometimes a cobalt glass is used to filter out the yellow of sodium.

·  The test cannot differentiate between all elements. Several metals produce the same flame color. Some compounds do not change the color of the flame at all

Flame Test Colors:

Symbol / Element / Color
As / Arsenic / Blue
B / Boron / Bright green
Ba / Barium / Pale/Yellowish Green
Ca / Calcium / Orange to red
Cs / Cesium / Blue
Cu(I / Copper(I) / Blue
Cu(II) / Copper(II) non-halide / Green
Cu(II) / Copper(II) halide / Blue-green
Fe / Iron / Gold
In / Indium / Blue
K / Potassium / Lilac to red
Li / Lithium / Magenta to carmine
Symbol / Element / Color
Mg / Magnesium / Bright white
Mn(II) / Manganese(II) / Yellowish green
Mo / Molybdenum / Yellowish green
Na / Sodium / Intense yellow
P / Phosphorus / Pale bluish green
Pb / Lead / Blue
Rb / Rubidium / Red to purple-red
Sb / Antimony / Pale green
Se / Selenium / Azure blue
Sr / Strontium / Crimson
Te / Tellurium / Pale green
Tl / Thallium / Pure green
Zn / Zinc / Bluish green to whitish green