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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Fire creates the type of environment in which more fire is likely to occur. This is an example of a *positive* feedback loop. Positive feedback tends to cause runaway effects ina system
Fire is a group of chemical reactions which require high temperatures to get started but then give off more energy then it takes to start them. This additional energy from the reactions are able to heat up nearby chemicals which cause them to undergo similar reactions themselves. This is how the fire spreads.
Fundamentally “burning” is something falling apart and releasing the energy that was holding it together. Think about taught ropes snapping and then flying around. The input to break those bonds is energy and since you’re releasing stores energy in this reaction, it can kick off the same reaction in things nearby if they also only need energy to react the same way which releases more energy, etc…etc… The reaction generates it’s own input in a cycle.
This is very different from a reaction that requires material as an input, like an acid for example. In that case the reaction uses up its inputs and doesn’t release more of that input so it will fizzle out and stop as soon as the original input has all undergone the reaction and been used up.
There reactions with materials that do generate more of the input material, but most often you’re getting into things like proteins copying themselves or other really complex molecular interactions.
An existing fire produces heat, which can ignite a nearby flammable material. That material will now be producing heat, because it’s on fire, and that heat can ignite a nearby material. That material will now be producing heat, because it’s on fire…
To address the “spreading” facet specifically; fire is generally a reaction between a fuel and oxygen, and since oxygen is everywhere, the reaction can spread pretty much indefinitely. Most other chemical reactions you encounter deal with very limited and localized reactants, they’re not being supplied by the atmosphere itself.
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.
Absolutely other reactions can and do spread. The major risk of fire is that the reactants (the chemicals that participate in the reaction) are *right there* out in the open and mixed together.
We live in a world where practically everything (except concrete) is made out of many flavors of hydrocarbons, or molecules that contain carbon bonded to hydrogen.
We also live in a world where oxygen gas is just floating around everywhere. The hydrocarbons have electrons they want to give up and oxygen is looking for electrons to take. And they are right there, next to each other, like two shy single people with so much in common but just need to make the first move.
So the world is a beaker and the 2 chemicals have been mixed in there together. That is why it can grow so large.
Thankfully there is a barrier to the reaction happening called “activation energy”. This is what stops the reaction from happening (and why you, your house, your lawn, and all of your plastics aren’t bursting into flame right now). Now on the flipside, the reaction makes a lot of energy which is more than enough to overcome this activation energy. Thats why it spreads.
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