How Flame Test Colors Are Produced

The colors in the flame
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 metal ions yield flame colors. Also, some metal ions display colors that are similar to each other. Nevertheless, the test is still useful for identifying numerous metals and metalloids.

Heat, Electrons, and Flame Test Colors

The flame test is 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 then 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 their 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 larger atoms is lower in energy than the light emitted by smaller atoms. So, for example, strontium (atomic number 38) produces a reddish color, while sodium (atomic number 11) produces a yellowish color.

The sodium ion has a stronger affinity for the electron, so more energy is required to move the electron. When the electron does move, it reaches a higher state of excitement. As the electron returns to its 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 during the flame test, while copper(II) emits green light.

A metal salt consists of a component cation (the metal) and an anion. The anion can affect the result of the flame test. For example, a copper(II) compound with a non-halide produces a green flame, while a copper(II) halide yields a blue-green flame.

Table of Flame Test Colors

Tables of flame test colors try to describe the hue of each flame as accurately as possible, so you'll see color names rivaling those of the big box of Crayola crayons. Many metals produce green flames, and there are also various 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 that you know what color to expect using the fuel in your laboratory.

Because there are so many variables involved, the flame test is not definitive. It is merely one tool available to help identify the elements in a compound. When conducting a flame 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-green

Phosphorus

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, palladium, and other elements do not produce a characteristic flame test color.

There are several possible reasons for this, one of which may be that the thermal energy isn't sufficient to excite the electrons of these elements enough 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 to natural gas, which tends to burn with a clean, blue flame. There are even filters that can be used to subtract the blue flame to view the flame or blister test result.