TED-Ed Video Lesson Transcript:
Sitting around a campfire, you can feel its heat, smell the woody smoke, and hear it crackle. If you get too close, it burns your eyes and stings your nostrils.
You could stare at the bright flames forever as they twist and flicker in endless incarnations.
But what exactly are you looking at?
The flames are obviously not solid, nor are they liquid. Mingling with the air, they’re more like a gas, but more visible — and more fleeting.
And on a scientific level, fire differs from gas because gases can exist in the same state indefinitely while fires always burn out eventually.
One misconception is that fire is a plasma, the fourth state of matter in which atoms are stripped of their electrons.
Like fire and unlike the other kinds of matter, plasmas don’t exist in a stable state on earth. They only form when gas is exposed to an electric field or superheated to temperatures of thousands or tens of thousands of degrees.
By contrast, fuels like wood and paper burn at a few hundred degrees —far below the threshold of what’s usually considered a plasma.
So if fire isn’t a solid, liquid, gas, or a plasma, what does that leave?
It turns out fire isn’t actually matter at all. Instead, it’s our sensory experience of a chemical reaction called combustion.
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. All of these are sensory clues that a chemical reaction is taking place.
What differs about fire is that it engages a lot of our senses at the same time, creating the kind of vivid experience we expect to come from a physical thing.
Combustion creates that sensory experience using fuel, heat, and oxygen. In a campfire, when the logs are heated to their ignition temperature, the walls of their cells decompose, releasing sugars and other molecules into the air.
These molecules then react with airborne oxygen to create carbon dioxide and water. At the same time, any trapped water in the logs vaporizes, expands, ruptures the wood around it, and escapes with a satisfying crackle.