How do things “burn”?
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While many things can burn and they vary in exactly how that reaction plays out, it’s usually a reaction where oxygen in the air rips its way into the molecules and replaces whatever the atoms were attached to with itself.
Long chains of carbons are broken up and each carbon pairs with two oxygen atoms, for instance.
This process releases a lot of heat, and often a fair amount of gas too.
Oxygen is a molecule that reacts easily with a lot of other things, combining itself with other molecules and releasing energy as heat when it does so. The process is known as oxidization, or generally burning. The process requires a fairly high temperature to start, but since it produces heat it tends to sustain itself once started. This is loosely what we call “burning”, and a “fire” is really just seeing the effect of extreme heat.
It’s a question of how easily oxygen can intrude itself into existing molecules. Different molecules are more open to the idea of coming apart and re-assembling themselves with oxygen. Hydrogen, methane, and other hydrocarbons famously burn fairly easily. Many organic chemicals will burn at high enough temperatures. On the flip side, water doesn’t like the idea because it’s essentially already the result of burning, since hydrogen + oxygen => water which was a burning process.
burning in a conventional sense of something like gasoline or a wood log or paper, or, more unfortunate living creatures sometimes, is a type of chemical reaction known as oxidation.
Oxygen is a fairly reactive element, even in the gas form around you that you breathe in. At high enough temperatures and when given something that can kickstart a reaction(like a spark of electricity), the oxygen in the air can break apart the bonds of a fuel source like the aforementioned, which have a lot of carbon and hydrogen atoms in their molecules, and recombine those atoms into carbon dioxide, water, and a lot of heat which keeps the reaction going. Smoke from fire is the result of some of that fuel source not being able to burn properly, either because not enough oxygen got to it so bits remain, or because the fuel has other elements in it besides just hydrogen and carbon that don’t burn.
As a chemist when I look at a campfire I imagine what I am seeing is essentially trapped sunlight being released. A tree takes CO2 which isn’t super useful to make things out of and uses the energy from sunlight to rip the oxygen’s off and combine the carbon with things like hydrogen and nitrogen to make long strands of things like cellulose which are good for building very large structures like trees. Burning is just that process in reverse. It’s like building with Legos. You can build very large things from smaller pieces, but they tend to want to break apart into smaller bits again if you drop them. Adding heat to wood (cellulose) is essentially the same as dropping your big Lego thing.
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