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We all know that the Ancient Greeks thought everything in the universe was made up of four indivisible elements: Earth, Air, Water and Fire. And we all snigger superciliously at their ignorance, because we all also know that Earth and Air are mixtures, pure Water is a compound, and Fire is – wait, what is Fire?

If you were ever curious enough to ask that question, there is a significant chance that you received one of the following incorrect answers:

1. Fire is pure energy. I always found it pretty hard to wrap my head around this idea. I mean, what exactly is “pure energy”? Mass times the speed of light squared?

Well, okay, I suppose light and heat could plausibly be suggested as being forms of “pure energy”, but they both can be understood in terms that go a bit further that just “pure energy”. Light is often well described as an electromagnetic wave, and heat can be described as the stored kinetic and potential energy of atoms and molecules. In contrast, if we called fire “pure energy” and just left it at that, we would really just be saying that we had no idea what it was.

2. Fire is a plasma. This answer isn’t actually necessarily wrong – fire can create a plasma. However, the fires most of us think of when we ask the question (candle flames, forest fires, burning buildings, Molotov cocktails, etc) almost never do create a plasma. You can be pretty sure of this because these ordinary fires are not affected by electricity or magnetism. A plasma – an ionized gas – would be affected by both.

Now that we’ve got those out of the way, let’s look at the correct answer to the question What is Fire?

Fire is a mixture of incandescent matter. 

Nice, simple, one-line answer, isn’t it? I wanted to give you that right up front, so you don’t get a little distracted by some of the complicating details we’ll go into next.

THE INGREDIENTS OF FIRE

Here’s the first of them. A fire can exist only in the presence of these four ingredients: heat, fuel, oxygen and a chain reaction. The fires you see around you are nearly all created during a combustion reaction between an organic compound (the fuel – an example would be the butane in your lighter) and oxygen. However, these reactions don’t usually start spontaneously – you need to provide heat to the fuel-oxygen mixture to get them started. Once the reaction gets started though, it often releases enough heat to keep itself going until all the fuel/oxygen is used up. Thus, a chain reaction keeps the fire going.

The four key ingredients in any fire are eloquently summed up by the following diagram, called the fire tetrahedron:

We now know much more than we did when we first asked the question. Here’s a quick interim summary: Under special conditions, a fuel/oxygen mixture reacts in a self-sustaining way to release incandescent matter (both gases and un-combusted solids like soot) that we perceive as fire. There’s just one last thing we need to clear up: what exactly does incandescent mean?

INCANDESCENCE

Once again, let’s keep things simple. We’ll start with the fact that anything that has a temperature above absolute zero (0 Kelvin, or -273 degrees Celsius) is emitting electromagnetic radiation in a process called thermal radiation. Why? Because all matter consists of charged particles (e.g. electrons and protons), and when you accelerate a charged particle, it gives off electromagnetic radiation (see Larmour Formula).  So who’s accelerating the atoms? The temperature is – when a body gains heat, its atoms/molecules begin to move about randomly (in fact, this is part of the definition of temperature), bumping into one another, and thus causing acceleration of charges.

Right, so everything around you is emitting thermal radiation in some area of the electromagnetic spectrum. The point I’m trying to get to is that some of that thermal radiation is in fact sending out visible light – and that is called incandescence. Tungsten filament bulbs work by heating tungsten to the point where its thermal radiation is in the form of visible light – i.e., by incandescence. (By the way, fluorescent lights work in a very different, and fascinating way, but let’s save that story for later). The flames in ordinary fires also give off light through incandescence.

Well, there you have it. Fire is a mixture of incandescent matter. You may now go back to mocking Aristotle.

Oh, one last thing, though: a little treat for having stuck around this long. Below is an image of a candle in space. As you’ll notice, it’s flame is pretty different from the ones we’re used to. It’s perfectly spherical, because in microgravity, there’s no “up” for the hot gases to go to, and they spread equally in all directions.

In a microgravity environment, a flame is spherical in shape