Why are flames blue?
Flames are blue because of the molecules that are produced during combustion. CH and C2 radicals give off light in the green to blue region of the visible spectrum (~435 nm for CH, and 470 & 515 nm for C2). The color can change if something other than a carbon based gas and air are burnt. For instance, a hydrogen-oxygen flame will give off light in the ultra-violet region. Addition of metals will also influence the color of the flame; such as, sodium creating a yellow flame, or copper a green flame. Finally, the temperature of the flame will give some change to the color, but in this case the light comes from hot soot particles rather than chemical reactions. For a cool flame the soot will give the flame a orange-red color, while a hot flame gives a yellow or whitish appearance. For more information on this look at Plank radiation, or Black Body emission.
