Blue flames arenít always hotter than yellow flames, because the color of light emitted by the flame can depend on exactly which atoms and molecules are in the flame. Each atom or molecule has certain special frequencies (colors) at which it absorbs and emits light, just like a musical instrument has special frequencies at which it absorbs and emits sound. Sometimes thatís more important than the temperature of the flame in setting the color. Some chemicals burn with a blue color, for example, so that if you burn some of these on an ordinary fire it will look blue for a while (some fireplace logs may do this). This does not mean that the temperature of the whole fire went up, just that these chemicals made the color change. For more information on this and on why many common fuels burn yellow, check out this answer
However, there are cases that follow the simple pattern you ask about, where the flame color changes smoothly from yellowish to bluish as it gets hotter. Simple burners fueled by oxygen and propane typically behave this way.
The key to making sense of this turns out to be that the energy emitted into the light comes in little packets, called quanta. High-frequency (bluish) light has high energy quanta and low-frequency (reddish) light has lower energy quanta. Temperature measures how much thermal energy is available to go into vibrating particles, etc, including the particles emitting the light. If the typical thermal energy of a particle is large compared to a quantum of light of some color, that color of light is easily emitted. But if the energy quantum is bigger than the typical thermal energy scale, those quanta hardly ever come out. So as you heat something up, first the lower energy (red) quanta show up, then also middle energy (say green), and finally theyíre joined by blue quanta.
This process makes no sense in classical physics, where there's no packet-size for light waves, so it provided the first key to the modern physics of quantum mechanics.
The actual color you see is set by the mixture of different light frequencies. Orange or yellow flames have fairly high wavelengths (low frequency) - most of the light being produced is actually in the infrared range, which we canít see. Blue-ish flames have much lower wavelengths (high frequency) with a lot of the light off towards the ultraviolet range, which we also canít see.
When a flame glows white, its temperature is somewhere in between those two. White is what you get when you have all the visible colors mixed together in about the same ratio as sunlight. So a flame with a temperature about the same as the surface of the sun looks white, if there arenít any chemicals in it which emit any special colors especially easily.
-Tamara & Mats & Mike
(republished on 07/24/06)