What makes fire “hot”?


Would in theory be possible to have a similar reaction that burns something without releasing heat?

In: 0

There are plenty of chemical reactions which remove heat rather than releasing it, they are called endothermic reactions. But the way combustion is defined limits it to specific kinds of reactions that release heat (called exothermic reactions).

It takes energy to break the chemical bonds between atoms in a molecule. Energy is also released when atoms come together to form bonds. If the energy that goes into a reaction is less than the energy that comes out, that makes it exothermic. Combustion is exothermic for this reason. The heat you put into it (to start the fire) is less than the heat that comes out as a result of the formation of new bonds between atoms as a result of the reaction.

Fire is a chain reaction, where the oxidation of one molecule releases energy, giving another molecule enough energy to react, and so on and so forth. As a product of the reaction, hot gasses are released.

This is the difference between iron rusting and a magnesium fire.

So a “cold fire” wouldn’t be a fire, but just room temperature oxidation, again like rusting. Or butter going bad.

Note, iron rusting still releases energy, it gets warmer, but it’s not a chain reaction, so it doesn’t rapidly spiral out of control.

Some other reactions actually consume heat, like a cold pack.