What is fire?

We know that fire results from the combustion of organic material and air, but don't you still sometimes wonder what fire is? Why are gas flames blue and wood-fire orange? And why do flames move in such a mesmerizing way? Chemistry may tell us the recipe for combustion, but the light show is all physics. When a flame burns cleanly (like a gas flame, blowtorch or the base of a candle), the heat excites the molecules to release light (usually pale-blue) from atomic transitions. That's quantum mechanics! Now when the fuel isn't as pure and doesn't entirely burn (like a wood or coal fire or the top of a candle flame), there's still some blue light, but you don't see it because it's overpowered by light from all the particles of soot & smoke - they're glowing red-hot! So why do hot objects glow? Thermodynamics. A process called "blackbody radiation" makes all objects glow with light of a color depending on their temperature. The reason that you don't see your friends glowing, though, is because we're too cool to glow with visible light - we glow in infrared. But lava, a hot piece of iron, or soot in a flame are all hot enough to glow with that familiar red-orange light. And the reason flames are shaped like tongues snaking skywards? Gravity! The earth's pull is what makes hot air rise, and this convection shapes flames into their familiar form. If you light a match in zero-G, the flame spreads outwards like a balloon - there's nothing to tell it which way to go, so it goes in all directions!

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