Flame tests are a very easy way to distinguish between ionic metal salts. Learning how to interpret their results is an important skill for chemistry students.
When energy is applied (in this case from the flame) electrons in the metal atom are excited from the ground state to an excited level. As the electron drops back down to the ground state – energy is released. This energy release is responsible for the colour observed in the flame test. Whilst it is a fairly rudimentary test, it can be very useful as a primary route for identifying metal salts.
The colour given off by each metal is somewhat unique to each element although it can be difficult to distinguish between similar colours and some elements may incur no colour change at all. Firework manufacturers take advantage of this – they use combinations of metal salts are to create dazzling fireworks in a range of colours. You should keep that in mind next bonfire night!
How to Conduct a Flame Test
To conduct a flame test, moisten a platinum wire with concentrated HCl and pass it through the flame. Repeat until the flame is colourless to ensure the wire has no contaminants on it. Then moisten some of your sample with the concentrated HCl and coat the platinum wire in it. Pass the wire through the flame and observe the colour released.
Common Metals and Their Flame Colours
Something to keep in mind
Sodium provides a yellow flame but unfortunately, is a common contaminant in many inorganic salts. You can use a blue piece of cobalt glass to filter out the yellow flame produced by sodium. If you see yellow flames, repeat the test but look through the glass. The cobalt glass will remove the yellow flame so that you can see any other colours more clearly. You may want to test a pure sample of a sodium salt to get an idea of the intensity of the yellow.
For Edinburgh students, you can find more information about flame tests in the Chemistry 2 Inorganic lab manual.