A flame test is used to identify metal ions based on their unique emission spectra. When the electrons in the ions absorb heat energy, they move to a higher energy level. They're unstable at a greater distance from the nucleus so they fall back, giving off the absorbed energy in the form of light. The color of the light emitted is related to the size of the energy change. When viewed through a prism, each element has a specific set of lines that it produces. All together the various wavelengths of the lines produce colors characteristic of each element. Here are some examples:
Sodium - yellow
Lithium - red
potassium - violet
strontium - red
calcium - red-orange
copper - blue-green
barium - green
The test is conducted by dipping a clean nichrome or platinum wire into distilled water then into a salt containing the element. Chloride salts are often used. Alternatively, the wire can be dipped into a dilute solution of the salt. It's then held in the flame of a lab burner to observe the color produced. If the same wire is used for more than one solution it needs to be cleaned each time in dilute hydrochloric acid to remove ions of the previous solution.
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