A question on the flame test

[Mrednaxe]

Hello everyone

 

I've recently conducted a flame test to find out some components in salts. http://en.wikipedia.org/wiki/Flame_test

 

Our teacher informed us that the flame test only indicate to what the positive ion in the salt could be. (These are salts with an elemental substance as positive ion)

 

The bunsen flame will change colour because some electrons in the positive ion will reach a higher orbit for a moment, and then fall back and in the process emit light of a specific energy (frequency).

 

My question is why this will not happen to the negative ion, I've discussed this with some friends but none have been able to give an definite answer.

 

Thanks for all replies:-)

[Mr Skeptic]

The alkali metals have a very definite emission spectra visible to the naked eye because they produce light at nearly exactly the same wavelength, so that you don't need to diffract the light into components to see what is there. I don't know about the alkaline earth metals, though. Most elements have emission spectra that are either not visible or of several colors so you couldn't tell by eye.

[John Cuthber]

A few metals that are not alkali or alkaline earth metals give colours too, copper is one example. Thalium is another but I don't think you will see that in a school lab.

In principle there's no reason why non-metals cant be identified by a flame test. The blue colour of a bunsen flame is due to a bunch of non-metals (excited CO molecules iirc) emiting light. However, it happens that none of the non metals emits much light in the visible region. The blue colour of burning sulphur might be thought of as en exception I guess but I don't think it's very useful.

If you are prepared to use some instrument rather than your eye to detect the light then a lot more chemicals can be identified by emision spectra.

[Mrednaxe]

Thanks Mr Skeptic and John Cuthber for the replies

 

So will the negative ion also emit light? (of some frequency)

 

An somewhat irrelevant spoiler is that the negative ions in these salts were composed of non-metal substances. I'd like an generic answer.

[Mr Skeptic]

Anything hot will emit light. If you can find the emission spectra information about a molecule, you should be able to identify it using a spectrometer, as well as find out what color it would look if it were visible.

[nitroglycol]

According to the Wikipedia page referenced above, the colour of a copper flame is variable, dependent on whether or not the copper compound is a halide.

[John Cuthber]

Copper atoms in a flame emit green light. Copper halide molecules (usually CuCl) emuit blue light.

Emisions from S2 and HPO (a couple of unusual molecules- but you often get funny molecules in flames) are used for detecting S and P compounds in chemistry.

http://www.shsu.edu/~chemistry/FPD/FPD.html