Question: Why does burning magnesium produce white light, and copper green light?

Leonie Bole answered on 5 Mar 2018:

Hi Coco, that’s a fantastic question!

Burning little strips of magnesium was always one of the more exciting practical experiments to do in school. When the metal comes into contact with the flame, a combustion reaction occurs. The energy given off by this reaction causes a lot of heat and light to be produced – the white light that we can see! Something else that is important here is that, for combustion to occur oxygen must be present. As a result, when the magnsium metal reacts with the oxygen, magnesium oxide (MgO) is produced – this is the white powder that you see at the end of the reaction.

In the case of copper, something a little extra happens when the combustion reaction occurs. When the energy (flame) is supplied to the metal, the electrons in the outer valence shell are excited, just as occurs with magnesium! However, when the electrons return back to their lower energy state, this extra energy is released. As it is released, the energy corresponds to a specific wavelength which, for copper, we are able to observe in the visible region of the spectrum. So really when we see the colour green here, it is the energy being released by the excited electrons of burning copper that we can interpret as a colour!