In a chemical reaction, molecules (themselves composed of a set of atoms) are rearranged in new molecules by creating and/or removing bound between atoms. Creating/removing these bounds will either release or absorb some amount of energy. When the reaction release energy, it is an exothermic reaction. When it absorb energy it is an endothermic reaction. Fire is the energy released by some exothermic reactions, in the form of heat and light.

It actually doesn’t have to be hot

Short answer is that fire is hot because it produces more energy than is used to start the reaction. Fire is, of course, a chemical reaction. You can learn more here: https://sciencenotes.org/why-is-fire-hot-how-hot-is-it/

In one sentence: heat is the measurement of movement of particles, and fire is when particles move very quickly, therefore hot. This is a very very basic explanation of it

We decided to call what fire does to us “hot” and what ice does to us as “cold”. They are physically both different manifestations of the same thing, temperature.

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