When burning a salt, how do we know the flame color comes from the metal ion and not the non-metal ion?I did a lab on this by burning different salts and observing the different flame colours. This...

When burning a salt, how do we know the flame color comes from the metal ion and not the non-metal ion?

I did a lab on this by burning different salts and observing the different flame colours. This was one of the analyze questions.

 

Your help will be very much appreciated!

Asked on by lalala07

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ndnordic's profile pic

ndnordic | High School Teacher | (Level 2) Associate Educator

Posted on

When you are doing a flame test you are using a solution of positive metal cations and negative metal anions.  The reason a color is observed is that during the flame test the positive ion can reacquire an electron, becoming a neutral element again. The electron that is gained is in a higher energy level and when it drops to a lower energy level a photon of light is emittted. The wavelength of the light emitted determines the color.

Since anions already have an extra electron, they would have to gain even more energy so the electron could then drop to a lower energy level so a photon could be emitted.  As this does not happen, the anion does not generate a color.

You could also test this by using the same metal ion with a variety of different anions and see that the same color is produced in each case.

william1941's profile pic

william1941 | College Teacher | (Level 3) Valedictorian

Posted on

You don’t exactly burn a salt. When you heat the salt it splits into the metallic and non-metallic ions.

Due to the heat the metallic ion changes state and when it returns the electromagnetic radiation given off has a particular color. For a compound, it is always the metal that is involved in determining the color that is present, the non-metal atoms have no contribution here.

For evidence you will find the color yellow given off when all sodium compounds are used, all strontium compounds give red, all potassium compounds give a purple radiation and copper compounds give a color between green and blue.

It depends only on the metal in the compound.

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