Flame Test and Ions
Name: Martha J.
Date: Monday, January 20, 2003
I understand that one of the ions in a flame test cause
the flame color. But given a compound such as Copper Nitrate. How is a
student to know which of the ions caused the color? Or how can I as an
educator explain to them this situation so that they can understand?
Before the fact, and without any prior information, you would not know
which species produces the color in a flame test. However, if you observed
the same result with several copper salts: the nitrate, the chloride, the
sulfate, and if you used a spectroscope to see the wavelengths more reliably
than the eye can, you would infer that copper is the very likely cause of
the emission. Supporting that inference would be the lack of color of those
wavelengths if the corresponding acids of the anions were to be tested. This
is a good example of how scientists work to establish the likely cause of
Only the metal ions (the cations) are responsible for the colors in flame
tests. In the example you cite, the colors you see arise from excitation of
Although all elements will produce a spectrum if their atoms are heated
sufficiently, not all will produce colors visible to the eye under
conditions usually accompanying an ordinary lab flame test.
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Update: June 2012