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Combustion, in chemical terms, is a reaction with a certain molecule and oxygen, and it produces, energy, $\ce{CO2}$, and $\ce{H2O}$ (That's how I learned it at least). So what makes a fire'smokey'? I had always thought that smoke was simply $\ce{CO2}$, but read that it was actually the result of incompletely 'combusted' material, yet I don't fully understand what this means. Is smoke made from material that starts to combust but stops before it finishes, or does it never even begin combustion? What would the chemical reaction be of a wood (Or other similar) fire that produces a lot of smoke? And finally, is there a certain element or compound that gives smoke its 'smokey' smell?

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Smoke is essentially aerated ash, unburned organics (including charcoal) and steam. You have to remember wood is not a pile of cellulose, but the remains of a dead organism. All the essential minerals that the plant needed for life are still in the wood such as calcium, phosphorous, sodium and potassium (namely potassium). When the organic matter is fully combusted, water and carbon dioxide are produced and the other elements are left as ash which can become aerated. When the water vapor reaches the cool air it condenses into a visible steam. However in real life wood is not fully combusted, but rather many unburned organic compounds due to a limited amount of oxygen, which give it its smell and fine aerated particles of charcoal which is scentless but very dark. This presence of unburned hydrocarbons in smoke is the cause of backdrafts in firefighting and can be demonstrated by lighting the smoke of a candle that has been blown out.

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  • $\begingroup$ Thanks @A.K. I guess this is as close as I can get to a chemical formula :) Also, thanks for the edit on those symbols, I'm still figuring out this stack exchange so I'm not sure how to do that yet :) $\endgroup$ – N A Feb 15 '16 at 14:38
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Fires need oxygen to burn properly. By that I mean if there isn't sufficient oxygen supplied to the fire, the material that is burning does not completely burn up. They come to the middle of the combustion reaction but they run out of oxygen. These bits of incomplete combustion are called soot or smoke. $\ce{CO2}$ is another by-product of combustion. There are particular woods that give off certain smells and flavours like hickory, but smokey smells are probably only caused by carbon.

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  • $\begingroup$ Carbon does not have a smell at all. The smell must be caused by some mixture of organic substances, too complicated (and too carcinogenic) to look into. $\endgroup$ – Ivan Neretin Feb 14 '16 at 19:26
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    $\begingroup$ @IvanNeretin I'm sure it's not the only one, but guaiacol is one of the major odor compounds in wood smoke. The purified stuff has a remarkably potent and persistent aroma. $\endgroup$ – hBy2Py Feb 14 '16 at 19:36

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