The heat of the flame excites the electrons of the metals ions, causing them to emit visible light. Every element has a signature emission spectrum that can be used to differentiate between one element and another.
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 blue 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.
Because of the limitation, the flame test might be used to rule out the identity of an element in a sample, rather than definitively identify it. Other analytical procedures should be conducted in addition to this test.
