How can fire spread? Isn’t it just a chemical reaction? What about other reactions? Is there a reason they don’t spread?
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In chemistry what we commonly call fire is broadly known as a reduction oxidation (redox) reaction. All of these reactions involve the freeing of atoms or electrons (oxidation) and the bonding of those atoms or electrons to new atoms (reduction). It takes energy absorption to oxidize something. When something is reduced it releases that energy. If you add enough energy to something you can oxidize it. Oxidation makes it unstable to it immediately reduces, forming a new bond typically with air. That immediate reduction releases more energy which causes more oxidations, which in turn causes more reductions until all the material is oxidized or reduced.
Some atoms form very strong bonds with other atoms and are nearly impossible to oxidize or reduce. Others form bonds with strength somewhere in between so they are hard to break but can release enough energy to sustain only their own reaction.
(Past ELI5 but more comprehensive explanation)
So when we burn wood paper or other natural fibers we are using a small amount of energy to oxidize the fiber. That breaks off carbon and other atoms from the molecule. The carbon and other atoms are unstable on their own so they are almost instantly reduced with the gases in the air to produce CO, CO2, and other products. Part of the reduction releases more energy into the fiber which oxidizes more carbon which is in turn causes a reduction that releases more energy. As long as there are atoms to oxidize and enough energy released in the reduction, the fire will continue to burn.
Not all molecules and atoms are capable of being oxidized or reduced. Some form very strong stable bonds that are very hard to break. Others form bonds which are somewhere in between. So some oxidation reduction reactions only produce enough energy to sustain themselves. For example thermite. A simple burning piece of wood isn’t actually enough to ignite a thermite reaction. You need something much more intense like burning magnesium. That heat is enough to kick off the reaction. In fact you can light magnesium with a stick, that kicks off an reduction oxidation reaction of higher intensity which can then be used to kick off a reduction oxidation reaction of the thermite.
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