Where do the atoms go?

January 22, 2025 7:27 AM

Say I burn a small piece of wood

until it is reduced to ashes, or possibly charcoal. Are the atoms and/or molecules of the original piece of wood released into the atmosphere, or are they rearranged into ash/charcoal residue?

posted by Czjewel to Science & Nature (7 answers total)

A little of both. Combustion is a process where carbon in the cellulose and other materials is reduced to carbon dioxide / monoxide (gas). Other materials do not transition to gas state, and are left behind.
posted by nickggully at 7:38 AM on January 22 [7 favorites]

Unburned wood is a mixture of many things, but mostly hydrogen, carbon and oxygen atoms mixed with water. Burning boils out the free water, but also causes a chemical reaction that releases various atoms as CO2 and water vapor. The residual ash has a much higher proportion of carbon.

All the atoms are conserved, though, since there isn't a nuclear reaction. They either remain as ash or are released to the atmosphere.
posted by Huggiesbear at 7:38 AM on January 22 [1 favorite]

Some of both, in terms of the atoms at least. In terms of the molecules, they are undergoing oxidation (which is why fire needs oxygen to burn) and being converted into all sorts of other molecules. If the organic compounds in the wood could oxidize fully to completion, you'd be creating mostly carbon dioxide and water, but a lot of them are not fully oxidized, leaving you with smoke, ie the stuff released into the atmosphere, and ash/charcoal.
posted by solotoro at 7:40 AM on January 22 [1 favorite]

[wiki:] the burning of vegetation releases nitrogen into the atmosphere, unlike elements such as potassium and phosphorus which remain in the ash and are quickly recycled into the soil
posted by HearHere at 7:50 AM on January 22 [1 favorite]

This is the subject of a famous, classic series of lectures by Michael Faraday in the 19th century: The Chemical History of a Candle. It's available in various formats from Project Gutenberg.
posted by JonJacky at 8:17 AM on January 22 [7 favorites]

Assuming fairly complete combustion, 90%+ of the atoms in the wood are released into the atmosphere as water vapour and carbon dioxide (and small amounts of other molecules like carbon monoxide, all the various components of smoke of which some are volatile and some are fine ash particles, etc). Note that both the water vapour and carbon dioxide contain quite a bit of oxygen from the atmosphere (which is why you need a supply of air for your fire to burn). So there are many more atoms in the exhaust than there are in the wood itself.

The small remaining pile of atoms is ash, which is a mix of a lot of different things and also contains oxygen from the atmosphere.

The residual ash has a much higher proportion of carbon.

Wood ash has less carbon by weight than dry wood.
posted by ssg at 8:21 AM on January 22 [3 favorites]

Are the atoms and/or molecules of the original piece of wood released into the atmosphere, or are they rearranged into ash/charcoal residue?

If you weigh the original wood, burn it, and then weigh the ash, the difference is approximately* how much wood got turned into vapor, gaseous combustion products, and small airborne particles.

* I say approximately because the ash contains some oxygen and carbon (in the form of stuff like calcium carbonate and calcium oxide), which could have been captured from the atmosphere during combustion. But compared to the amount of oxygen and carbon released from the burning wood, this is not going to significantly change the overall picture.
posted by aubilenon at 8:43 AM on January 22 [2 favorites]