The Colour of Fire...

Another unknown person asks - “why is the bottom of flames blue, and what reaction is it that causes this?”

There are lots of colours of flames, stretching right through the visual spectrum of light, and some of it just outside the visual spectrum. We see light in the flames of fires because electrons are being excited into higher orbits, the energy needed to do this coming from other oxidative reactions happening about the local area.

You see, when things burn, they do not initially give out any light. The reaction with oxygen is very exothermic (energy is released from the system when the reaction takes place, the opposite to endothermic reactions.) The heat from these reactions passes through the electrons (this is actually what “vibrates” when things get heated, not the atoms/molecules themselves, hardly.) When this energy becomes sufficient (in various quanta/amounts), an electron is able to break free of it’s ground state, and jump into another orbital. This energy in the form of high energy photons (yes, a form of light, but more in the UV range) is re-released as a photon of lower energy, and a small part of the energy being released as actual heat. The released electron will be in a high blue state, and hence the blue colour at the bottom of a flame (of high enough intensity). These are recaptured by air molecules, and the carbon dioxide and water (etc), which may increase the orbital of these particles. When these are adsorbed, the electrons re-release the photons, at a lower energy state, and hence the gradual progression from blue to yellow and then to red.

Not all flames will have a blue base though. The initial input of energy, from the combusting material, needs to be sufficient enough to start the electron jump at a high enough energy state to produce blue level photons. The amount by which the electron falls in it’s orbitals is what determins the colour of light that will be emitted. This also gives each combusting material it’s own unique light pattern as each orbital in each atom is unique. Hence burning Potassium gives a liliac flame, copper it’s strange green flame, and complex hydrocarbons like pertol a yellowey white.

So it’s not a reaction at all that produces the light, it only initiates the sequence which causes light to be emitted.

Hope this has helped

Triga@ask-me-anything