Is fire a solid, liquid, gas or plasma?

Fire is certainly not a solid or liquid, but it is also not a gas or plasma.

Mixed with air, fire appears more like a gas. However, fire is different from gas because gas can exist in this state indefinitely while fire will always die out eventually.

Another misconception is that fire is plasma, the fourth state of matter in which atoms are stripped of electrons. Plasma only forms when gas is exposed to an electric field or heated to thousands or tens of thousands of degrees. Fire burns with fuels such as wood and paper at only a few hundred degrees, much lower than the normal threshold of plasma.

So what is fire?

Fire is actually not physical at all but is the human sensory experience of a chemical reaction called combustion.

In some ways, fire is like a sensory signal that a chemical reaction is taking place, like leaves changing color in the fall, the smell of fruit ripening, or the flickering light of fireflies.

The difference is that fire engages multiple senses at once, creating a more immersive experience. Fuel, heat and oxygen combine to create combustion.

When lighting a campfire, when wood logs are heated to combustion temperature, their cell walls decompose, releasing sugars and other molecules into the air. These molecules will react with oxygen in the air to produce CO2 and water.

At that time, the water standing in the logs evaporates, expands, cracks the surrounding wood and then escapes, creating a popping sound. When the fire heats up, CO2 and water vapor produced during combustion also expand and rise into a thin column. Gravity causes the flames to have their characteristic bamboo shoot shape.

Without gravity, molecules cannot separate due to density. At that time, the flames will have a completely different shape.

Humans can see all of this because combustion also produces light. The color of light produced by combustion depends on the temperature of the molecules. The hottest flames are white or blue.

The color of fire can also be affected by the type of molecules in the fire. For example, unreacted carbon atoms from wood logs form small pieces of soot that fly into the fire, emitting orange-yellow light. Substances such as copper, calcium chloride, potassium chloride, can add their characteristic colors to fire.

When burning, fire also continues to generate heat. Heat maintains the fire by keeping the fuel at or above its ignition temperature.

But eventually, even the hottest fires will run out of fuel or oxygen and disappear with smoke like they were never there.

Samuel Daniel

Update 10 June 2024