Explain why a metallic ion produces a characteristic color in a flame test, regardless of the compound used as the source of the ion.

As a metallic ion is heated in the flame test the valence electron are excited or pushed to higher energy level. As the electron get excited they tend to move back to stable cell. while moving back to stable cell it emits energy in the form of light which is what we see in flame tests. Thus metallic ion will emit its characteristic colour no matter which compound it is form. That is, Cu2 + ion will always produce brick red colour regardless of its compound, because it is just metallic ion that get excited not the entire compound.