While the Encyclopaedia Britannica definition is pretty poor and Cecils version is a clear improvement (apart from the “in the presence of heat” bit), surely the real point is what do you see when you’re looking at the flames?
For the classical yellow/orange flame the answer is mainly solid particles (soot) at ± 900 degrees C. The colour of light emitted from a body is a function of it’s temperature and is commonly referred to as black body radiation, interestingly it was this observation that lead Max Planck to come up with the concept of Quanta and kicked off the whole Quantum Mechanics malarkey.
For the classical yellow/orange flame the answer is mainly solid particles (soot) at ± 900 degrees C. The colour of light emitted from a body is a function of it’s temperature and is commonly referred to as black body radiation, interestingly it was this observation that lead Max Planck to come up with the concept of Quanta and kicked off the whole Quantum Mechanics malarkey.
One part of Cecil’s column I didn’t follow was this:
So far as I know, the color of the incandescent glow is a function of temperature. Is he saying that an Indy car fuel fire burns too hot to be in the visible spectrum? And if that’s it, surely charcoal would, if anything, burn too cold.
No, the color of the visible flame is really the color of the particles in the flame that are glowing based upon their temperature. Clear flames are flames that do not have unburned fuel or soot in them. The energy itself does not glow.
If a flame is the result of the chemical reaction of heat, fuel and oxygen and is termed ‘rapid’, compared with oxidation and digestion, is an explosion even more rapid or even instantaneous combustion? Hello members, looking forward to spontaneous chats.
An explosion is a rapid expansion of pressure due to rapid combustion. Typically the combustion for an explosion is of the type that produces large volumes of gas from small volumes of fuel. Thus the gas expands, pushing outward, causing the outgoing pressure wave we call an explosion.
Something I’ve wanted to ask but was too small to start a thread on: does the material affect the radiation/colours produced? Why use thorium in mantles and lime in limelight? Is this also why some metals melt before glowing, and some glow before melting?
Yes, flame color is affected by the material being burned. I had a Jr High science experiment where we burned different substances in a bunsen burner flame and then identified the substance based upon the color of the flame.
[aside]That is trickier than you might think if the two partners are both color blind.[/aside]
Thorium is used because when heated to incandescence, it glows more visible light than infrared light.
Basically, the standard light bulb problem. Incandescent bulbs (i.e. the standard light bulbs being phased out for fluorescent and LED bulbs) are filaments of material that when heated via electric current incandesce, i.e. put out visible light. The problem is that they also put out infrared, i.e. heat. As someone once described them, incandescents are heaters that emit light. Energy efficiency strives to find ways to emit more light and less heat.
Rare earth metalic oxides seem to have good light to heat ratio.
A very complicated way of saying that apparently certain materials, when they burn, break into particles that get hotter than the surround air in the flame, and thus emit a higher frequency than expected by a black body at the temperature of the air in the flame.
There is debate over whether this factors into latern mantles.
Limelight uses quicklime (i.e. calcium oxide) specifically because it is the first material found to incandesce when heated. Thomas Drummond saw the effect and realized it would be useful in surveying, and developed the effect into a light source. Calcium oxide could be heated with a flame enough to glow without melting.
I can’t definitively answer why some materials melt before glowing and some glow before melting. I suspect it has to do with the material’s melt temperature (i.e. the molecular bond strength) vs the black body radiation temperature (i.e. the temperature when thermal emission is in visible wavelengths). But I’m no expert.
I’ve seen thin copper sheet actually burn with green flames on the application of heat from a blowtorch, with neither glow nor melt, initially. Thanks for your reply to my question Irishman, still not convinced on the sublimation of a solid to a gas, as in the case of a normally non flammable solid, ie; flour, if a handful were thrown in the air and a naked flame applied the suspended particles would explosively ignite. I too am no expert but have done this ‘trick’ several times over the years. Dust explosions were a common hazard in the early industrial age.
