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Name: Martha J.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: Monday, January 20, 2003


Question:
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?


Replies:
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 some observation(s).

Vince Calder


Martha,

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 copper atoms.

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.

Regards,
ProfHoff 555



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