The Science Of Fire

Author : Ananya Shukla

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? Is it a solid, liquid or gas?

The flames are obliviously not a solid nor a liquid. At first, It mat seem that fire is a gas, just more visible. And on a scientific level, fire differs from gas because gases can exist in the same state indefinitely while fire extinguishes out. One misconception is that fire is a plasma, the fourth state of matter in which atom are ionized. However plasmas don't exist in a stable from on earth. They only from if the gases are exposed to an electric field or superheated to thousands of degrees. By contrast, wood and other types of fuel only burn at a few hundreds of degrees. So if fire is not a solid, liquid, gas or plasma then what is it? Well turns out it is not matter at all, but a sensory experience of a chemical reaction called combustion. It is like a fruit as it ripens, or as the leaves change color. All of these are sensory clues of a chemical reaction is taking place. What differs about fire is that it engages a lot of sense at the same time, creating a 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 the cells decompose, releasing sugars and other molecules into the air. The 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. As the fire heats up, the carbon dioxide and water vapor created by combustion expand. Now that they're less dense, they rise in a thinning column. We can see all of this because combustion also generated light. Molecules emit light when heated, and the color of the light depends on the temperature of the molecules. The hottest flames are white or blue. Besides colorful flames, fire also continues to generate heat as it burns. This heat sustains the flames by keeping the fuel at or above ignition temperature. Eventually, 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.