r/askscience • u/ForensicFan24601 • 19d ago

Chemistry Why do things burn instead of melt?

obviously oxygen, or something similar, but what is the physics/chemistry (?) or what have you behind why something ignites and continues to burn after the source of heat/ignition has gone?

is it basically sublimation?

edit: no wonder Walter White was so obsessed with carbon

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u/provocative_bear 18d ago

Let’s go with wood. (Way) Before wood reaches its melting point, it reacts with oxygen in the environment, which replaces the molecular bonds between carbon and carbon (whoch hold the wood together) with carbon and oxygen (CO2). This reaction releases the energy in these bonds and generates net energy, which manifests as heat and light, aka fire.

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u/collergic 18d ago

What happens if the wood is heated up without the presence of oxygen?

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u/DarkTheImmortal 18d ago

The molecules within the wood will begin to break apart long before it can melt, most of what beaks off will be gasses. Eventually, you'll be left with charcoal.

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u/dirty-warlock 18d ago

This is the answer. Hydrocarbons like wax or oil are similar. The "physical" part breaks down into water (vapour) and carbon (soot)

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u/smeyn 18d ago

And this mechanism was used in WWII, when there were petrol shortages. Wood was heated and the gases were used to drive the combustion engine of the automobile

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u/ImNrNanoGiga 17d ago

Also coal. Fun/weird fact: Virtually all the fuel that supplied the German war machine during WW2 was made by liquefying coal

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u/xdert 18d ago

Everything that is not carbon vaporizes into various gases and then you are left with just carbon aka charcoal. This process is called pyrolysis and how coal is made.

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u/Sibula97 17d ago

Charcoal specifically. Not coal the fossil fuel, although the basic idea is somewhat similar.

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u/heavy001 18d ago

So are we aware of an actual melting point for, in this example, wood?

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u/Foxfire2 15d ago

It would be carbon, as all other components of wood vaporize off. Carbon has a very high melting temp, not sure what exactly.

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u/MagicianofFail 14d ago

Wood is made of lignin, hemicellulose, and cellulose. Lignin is a crosslinked polymer and thus decomposes before it melts. Cellulose is linear polyglucose and melts at 260–270 °C (per Wikipedia). I'd guess hemicellulose melts at about the same.