How Flame Test Colors Are Produced

Deciphering How Flame Colors Relate To Element Electrons

The colors in the flame test result from the movement of electrons in metal ions as they gain thermal energy.
The colors in the flame test result from the movement of electrons in metal ions as they gain thermal energy. Philip Evans, Getty Images

The flame test is an analytical chemistry method used to help identify metal ions. While it's a useful qualitative analysis test (and a lot of fun to perform), it can't be used to identify all metals because not all of their ions yield flame colors. Also, some metal ions display colors similar to each other. Have you ever wondered how the colors are produced, why some metals don't have them, and why two metals can give the same color?

Here's how it works.

Heat, Electrons, and Flame Test Colors

It's all about thermal energy, electrons, and the energy of photons.

When you conduct a flame test, you clean a platinum or nichrome wire with acid, moisten it with water, dip it into the solid you are testing so that it sticks to the wire, place the wire in the flame, and observe any change in flame color. The colors observed during the flame test are due to the excitement of the electrons caused by the increased temperature. The electrons "jump" from their ground state to a higher energy level. As they return to ground state they emit visible light. The color of the light is connected to the location of the electrons and the affinity the outer shell electrons have to the atomic nucleus.

The color emitted by the larger atoms is lower in energy than the light emitted by smaller ions. So, for example, strontium (atomic number 38) gives a reddish color as compared to the yellow color of sodium (atomic number 11).

The Na ion has more affinity for the electron, so more energy is required to move the electron. When the electrons does movie, it goes to a higher excited state. As the electron descends to the ground state it has more energy to disperse, which means the color has a higher frequency/shorter wavelength.

The flame test can be used to distinguish between the oxidation states of atoms of a single element, too. For example, copper(I) emits blue light in the flame test, while copper(II) produces a green flame.

A metal salt consists of a component cation (the metal) and an anion. The anion can affect the result of the flame test. A copper(II) compound with a non-halide produces a green flame, while a copper(II) halide yields more of a blue-green flame. The flame test can be used to help identify some non-metals and metalloids, not just metals.

Table of Flame Test Colors

Tables of flame test colors try to describe the hue of the flame as accurately as possible, so you'll see color names rivaling those of the big Crayola box of crayons. Many metals produce green flames, plus there are different shades of red and blue. The best way to identify a metal ion is to compare it to a set of standards (known composition), so you know what color to expect using the fuel and technique in your laboratory. Because there are so many variables, the test is only one tool to help identify the elements in a compound, not a definitive test. Be wary of any contamination of the fuel or loop with sodium, which is bright yellow and masks other colors.

Many fuels have sodium contamination. You may wish to observe the flame test color through a blue filter, to remove any yellow.

Flame ColorMetal Ion
blue-whitetin, lead
whitemagnesium, titanium, nickel, hafnium, chromium, cobalt, beryllium, aluminum
crimson (deep red)strontium, yttrium, radium, cadmium
redrubidium, zirconium, mercury
pink-red or magentalithium
lilac or pale violetpotassium
azure blueselenium, indium, bismuth
bluearsenic, cesium, copper(I), indium, lead, tantalum, cerium, sulfur
blue-greencopper(II) halide, zinc
pale blue-greenphosphorus
greencopper(II) non-halide, thallium
bright green

boron

apple green or pale greenbarium
pale greentellurium, antimony
yellow-greenmolybdenum, manganese(II)
bright yellowsodium
gold or brownish yellowiron(II)
orangescandium, iron(III)
orange to orange-redcalcium

 

The noble metals gold, silver, platinum, and palladium and other elements do not produce a characteristic flame test color. There are several possible reasons for this, one of which may be the thermal energy isn't sufficient to excite the electrons of these elements enough that they can transition to release energy in the visible range.

Flame Test Alternative

One disadvantage of the flame test is that the color of light that is observed depends very heavily on the chemical composition of the flame (the fuel that is being burned). This makes it hard to match colors with a chart with a high level of confidence. An alternative to the flame test is the bead test or blister test, in which a bead of salt is coated with the sample and then heated in a Bunsen burner flame. This test is slightly more accurate because more sample sticks to the bead than to a simple wire loop and because most Bunsen burners are connected

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Helmenstine, Anne Marie, Ph.D. "How Flame Test Colors Are Produced." ThoughtCo, Mar. 2, 2016, thoughtco.com/how-flame-test-colors-are-produced-3963973. Helmenstine, Anne Marie, Ph.D. (2016, March 2). How Flame Test Colors Are Produced. Retrieved from https://www.thoughtco.com/how-flame-test-colors-are-produced-3963973 Helmenstine, Anne Marie, Ph.D. "How Flame Test Colors Are Produced." ThoughtCo. https://www.thoughtco.com/how-flame-test-colors-are-produced-3963973 (accessed November 19, 2017).