Question: At home i have some birthday candles that have multi coloured flames, i have always wondered how they work. many thanks

Andrew Fensham-Smith answered on 23 Jun 2015:

The flames are from different metals used. For example, sodium gives a bright red flame, copper gives a blue flame, potassium gives a lilac flame and so on. The reason why they’re coloured is even more interesting though…

Colour comes about from either emission of light, or filtering of it. In a flame, it’s emitting light because you’ve made it really hot and it has a lot of energy. The specific colour of the flame depends on the energy of the light that comes off – higher energy light is blue, while lower energy is red. The amount of energy the light has depends on the difference between what I will call energy levels. Imagine jumping from the floor to a step – pretty small jump, doesn’t need much energy, so it’s a low energy move to go from the floor to a step. To go from the floor to your chair though means you have to jump a much larger space, and is a higher energy move. In elements, electrons act in the same way – they move between different energy levels. When you burn something, the electrons are going from a higher energy level into a lower one. In some elements, this move is large and a lot of energy, and so you get a blue flame (like in copper). In other elements this move is lower, and so you get light with a lower energy being given off (like in sodium).

Hard to explain, but I hope you get the gist!