What is fire?

So, here's a question: what actually is fire? Of course, everyone knows what fire is. We've known since the Stone Age, right?

Well, yes and no. We might think we know what fire is because it's so familiar. But scientifically, it's kind of hard to pin down.

Clearly, it's not a solid or a liquid. So, maybe a gas? The flames of a fire do involve hot gas products, which rise because they are less dense than the surrounding cooler air. But the flames we see are actually burning soot (burned particles of carbon) that glows yellow-orange because of the high temperatures. However, the flames only exist while the fire is burning, and don't exist in a stable state on their own. They can't be collected in a jar like CO2 or water vapour, and so they are not a gas.

How about plasma, the fourth state of matter? Plasma is like a soup of charged particles, electrons and ionized atoms. It's possible there might be enough ionized atoms in the hottest part of the most intense fires to qualify as a kind of weak plasma, but fire as a whole does not behave as a plasma.

In fact, it turns out that fire is not matter at all. Fire is a process, a kind of chemical reaction called combustion, which requires fuel (something to burn), oxygen, and an initial spark or heat source. Interestingly, it seems that fire - with visible oxygen-fuelled flames - is unique to Planet Earth (or so this Science Alert article claims, although this seems improbable to me).