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As I understand; the reason we have smoke in a fire is because the heat causes wood (or whatever material is burning) to dis-integrate, the disintegrated components react with oxygen to create more heat; causing a chain reaction.

Much of the smoke we see is the dis-integrated carbon in the air from incomplete combustion.

However, when we heat charcoal in a vacuum, there is no smoke. But if heat is what causes carbon to dis-integrate, then why does heating carbon (regardless of oxygen presence) not cause smoke?

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    $\begingroup$ Carbon doesn't disintegrate; wood does. $\endgroup$ – Ivan Neretin Jan 22 at 15:33
  • $\begingroup$ But then where does the carbon in the smoke come from when the wood burns? $\endgroup$ – user1543574 Jan 22 at 16:00
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    $\begingroup$ From pyrolyzed wood and resulting vapours and gases, containing carbon. Charcoal as carbon is the result of wood pyrolysis. As all has already been done, there is nothing to make a smoke. $\endgroup$ – Poutnik Jan 22 at 16:02
  • $\begingroup$ I see, can you confirm if this understanding is correct?: basically, when fire burns; all of the smoke comes from the components gassing off the wood, which separates into carbon from the heat. Some of the carbon reacts with oxygen to form light but not all of it. ---- However, the actual carbon in the wood only ever reacts with oxygen and therefore doesn't ever contribute to smoke. $\endgroup$ – user1543574 Jan 22 at 16:11
  • $\begingroup$ In short, smoke is drops and very little pieces. In vacuum nothing can remove particles from a surface, so even if carbon would behave as it does in air, there is no convection. Instead what happen to C in these vacuum conditions is interesting and I don't know much. $\endgroup$ – Alchimista Jan 23 at 10:47
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You say that wood decomposes under heating and produces gases(1) which then burn. That's correct. It would be wrong, though, to assume that all combustible materials are like that. Indeed, many - arguably, most of them are. Wax, tar, and all kinds of plastic come to mind. But carbon is a different story.

See, carbon is very heat-resistant, about as much so as the most heat-resistant thing you know, only more. You heat it to $1000\;^\circ\rm C$, and nothing happens. Well, it may start burning provided there is air around, but short of that, it is nothing. You heat it a few hundred degrees more, it starts glowing red, then white, then steel melts, then you pass $2000\;^\circ\rm C$ and continue to push on, then everything else melts, by now your carbon glows so bright that you can't see a thing, but it is still intact.

Burning of carbon, unlike burning of many other materials, is a true heterophase reaction. You chip atoms off that piece one by one, hence no smoke is produced. Burning of wood or plastic is another story. Combustible gases are produced and fly away, and when some of them do not burn completely(2), they leave a solid residue (which, BTW, may or may not consist mostly of carbon). That's what we call smoke.


P.S. Thought I'd rather add two clarification notes:

(1) When heated, plastic turns into gases all but completely. Wood doesn't. Much of it remains in the form of charcoal, which is mostly carbon and burns like carbon, that is, producing no smoke.

(2) Incomplete combustion of carbon is also possible, and it results in carbon monoxide, which killed quite a number of people over the past few millenia. But it isn't smoke; it's a gas. You can't see it. You just see people dropping dead.

So it goes.

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    $\begingroup$ I think you missed the point that when something is being burned that products other than just carbon dioxide and water are created. Burning wood creates hundreds if not thousands of different compounds. Some of what burn completely other which do not. You often also get particulate matter when burning something. // Also if you heat something in a vacuum then there is no oxygen to promote burning. Rather what you are seeing in that circumstances is thermal decomposition and out-gassing. $\endgroup$ – MaxW Jan 22 at 23:17

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