The shivering cold days are finally here, and you’ll pretty much enjoy being around a campfire. Its warmth, flickering light, crackling noise, and distinctive smells are enough to give you a sense of enjoyment. If you get too close, the heat will tingle your nostrils and its searing flames will burn you.

Well, the flames can be quite captivating to gaze into, but what exactly are you looking at? Of course, the fire. But is it a solid, or a liquid, or gases? No, it’s neither of them, nor is it widely held misconception “plasma”: the fourth state of matter (or a point) where the atoms lose all their electrons, leaving a highly electrified cluster of nuclei and free electrons.

Anyone with a good scientific knowledge would describe fire as a chemical reaction that requires – oxygen, fuel and heat, which are referred to as The Fire Triangle, or Combustion Triangle. But this doesn’t clearly explain what the fire is, or does it? This TED video based on a lesson by Elizabeth Cox has an even more friendly explanation.

Just by observing, one can easily say that fire isn’t a solid or a liquid, instead it sort of looks a lot like a gas.  But this doesn’t sound scientifically correct, too. Because unlike gas, which exists in the same state indefinitely, fire always burns out. Eventually.

The reason it is not plasma either is that plasmas don’t exist in a stable state on earth. They only form when electrons are ionized at an ultra-high temperature. So if the fire isn’t a solid, liquid, gas, or a plasma, what state of matter is it?

Turns out fire isn’t a matter at all. Instead it is our sensory experience of a chemical reaction called combustion. Fire stimulates a lot of our senses at a time that it makes us create a scenario we expect to come from a physical thing. Combustion causes that to happen using fuel, heat and oxygen.

“In a way, fire is like the leaves changing color in fall, the smell of fruit as it ripens, or a firefly’s blinking light”, the video explains. “All of these are sensory clues that a chemical reaction is taking place.”

When you heat a log of wood, sugar and other molecules are released into the air. These molecules then react with the oxygen to make carbon dioxide and water. During the process, water trapped in the logs vaporizes into steam. The steams then expand and rupture the wood around it, and escape making that crackling sound that most of us find relaxing.

And, as the fire continues, the carbon dioxide and water vapor keep expanding due to gravity, become less and less dense and rise in thinning column. This explains why flames look the way they do. In a place, like ISS, where gravity is lot less weaker, the flames behave and look significantly different.

“Without gravity, molecules don’t separate by density and the flames have a totally different shape.”

Combustion also causes molecules to generate light, and the color of light depends on the temperature and types of the molecules.

The hottest part of the flames burns in either white or blue, then it turns yellow-orange as it the temperature falls. Substances like copper, calcium chloride, and potassium chloride can also influence the color of the flame.

“Besides colorful flames, fire also continues to generate heat as it burns,” the narrator explains. “This heat sustains the flames by keeping the fuel at or above ignition temperature. Eventually, though, even the hottest fires run out of fuel or oxygen. Then, those twisting flames give a final hiss and disappear with a wisp of smoke as if they were never there at all.”

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