What causes the colours in flames?

FlameThere is something mesmerising about fire. Whether it is sitting around a log fire or staring into the dancing flame of a Bunsen burner, we’ve all gotten lost in the hypnotising motion and unique colours of flames. So what is it that gives a flame its spellbinding colour?

Well, it’s all about electrons, or more accurately, the movement of electrons between energy levels. Now then, without getting too technical, let’s talk quantum mechanics. (Relax! If that freaks you out, all you need to understand is that the “quantum” part of “quantum mechanics” is taken from the word quantity, or fixed quantity.) You could say a flight of steps have a “quantised” incline, whereas a ramp does not, and instead has a gradual incline. Electrons are very much the same as the flight of stairs in that they have ‘quantised’ energy levels – although the gap between each energy level (step) is not necessarily the same as it is in a flight of stairs.

Normally an electron of an atom sits at its lowest possible energy level, known as the ground state. (Well, technically it doesn’t ‘sit’, but whizz around the atom) An electron will move up a step to a higher energy level if it is provided with enough energy to reach that level. There are no half measures with energy levels; an electron either absorbs enough energy to rise to a higher level or doesn’t absorb it at all. There is of course, plenty of energy kicking about in a burning log or candlewick to step our little electron to the next energy level. Once in a higher energy level, or “excited state”, an electron will soon be longing to return back to the comfort of the ground state.

When an electron falls back down to a lower step, it emits energy as light. A photon of light is emitted that exactly corresponds to the size of the energy level step. This giving off of light energy is the opposite process of energy absorption. And because the energy gaps are quantised, the energy emitted between two levels, or steps, will always be the same. Many light photons are therefore produced with the same energy (and so colour). And this is what gives a flame its colour.

Different elements and bonds between atoms (formed when the atoms share electrons) have differently spaced energy levels and hence produce different colours: strontium burns with a red flame, copper with a green flame, and potassium with a beautiful lilac for example. Most carbon-based fuels, such as the gas in your stove, burn blue because many of the short-lived substances produced during combustion emit light in the blue region of visible light spectrum. Soot and other incompletely combusted products of wood burning are carried upwards with the heat and cause the yellow flame that we’re all familiar with.


 
More information can be found at:

http://www.chemicalconnection.org.uk/chemistry/topics/view.php?topic=3&headingno=5

And

http://www.dedoimedo.com/physics/flames-yellow.html

Answer by Thomas Donoclift

Photo Credit: Flame colours by scienceatlife, on Flickr


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Article by Thomas Donoclift

December 17, 2014

Thomas Donoclift is currently working towards his PhD in radiation chemistry at the University of Manchester, UK. He is based in sunny West Cumbria; close to the heart of Britain's nuclear industry, at the University’s Dalton Cumbrian Facility. Thomas enjoys music, board games, and subjecting aqueous solutions to harsh radiation fields. He's nice really.


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