Fire does not have its own state of matter and appears to be, as far as we know, a phenomenon exclusive to our planet.
Fire is an ancient technology that helped shape human evolution. Our ancestors used fire for safety, cooking and preserving food. They gathered around a flickering campfire to share stories, pass on cultural knowledge and build community.
Today, fire is an important industrial tool. It remains intrinsically linked to our daily lives and our rituals (think of blowing out the candles on your birthday cake). Just like millions of years ago, fire can shape our landscapes, having the power to both devastate and revitalize entire ecosystems.
Fire is so familiar, and yet it can be difficult to define. What, after all, is fire?
Let’s start with a question that’s a little easier to answer.
What are the ingredients of fire?
To start a fire, we need three things: fuel (something to burn), oxygen, and an initial spark or heat source. This is known as the fire trianglebut you can also call the fuel and oxygen “reactants” and the initial heat “activation energy”.
In a forest fire, organic matter (such as wood) provides the fuel. Oxygen is available in the air, and activation energy can come from different sources, such as lightning or human activities.
If we remove one of the reactants, the fire cannot continue to burn. To extinguish a forest fire, heat can be removed by pouring water on the flames. The water turns into vaporwhich also smothers the fire by displacing air. The fuel can be consumed by the fire itself or removed preventively through controlled burns or cultural burns.
The main “product” of fire is energy, along with the gases carbon dioxide and water vapor. When there is more fuel than oxygen for combustion, as occurs in a forest fire, additional products may emerge. One of them is the sootwhich consists of tiny, partially burned carbon particles. These products interact to produce what we feel and see when we experience fire.
The heat we feel from fire comes from the energy that radiates outwards in the form of heat. The hot gases produced rise because they are less dense than the surrounding cooler air. The gases carry with them soot particles that glow orange-yellow due to their high temperature.
In a forest fire or bonfire, it is the incandescent soot that we perceive as flames. The flames, in fact, extend far beyond our field of vision. As the soot rises, it cools and emits light in colors we can’t see, like infrared light.
So what is fire?
Obviously, it is neither a liquid nor a solid. Although flames involve hot gases, they only exist while the fire is burning. They do not exist in a stable state on their own and we cannot collect flames in a container as we could with CO₂ or water vapor. Then, the flames and fire are not gases.
We can also rule out plasma – the fourth state of matter. Plasma is similar to an extremely hot gas, but with some important differences.
A plasma contains so much thermal energy that the atoms that make it up ionize, which means that they can no longer retain all of their electrons. Plasma is like a soup of charged particles, both electrons and ionized atoms, that can conduct electricity and respond to a magnetic field.
In the hottest parts of the most intense fires, it is possible that there are enough ionized atoms to form areas of weak plasma. However, plasma is not stable on its own and fire, as a whole, does not behave like a plasma.
In fact, fire is not matter. Fire is a process. It’s a type of chemical reaction called combustion.
A process unique to Earth
Gases and plasma are everywhere in the universe, but fire as we experience it – with visible flames fueled by oxygen – seems be exclusive to Earth.
Earth itself formed from dust and gas around a young Sun, which is so hot that it is almost entirely composed of plasma. The universe is home to trillions of galaxies, each filled with stars and possible planetary systems, so there’s a lot of gas and plasma out there.
However, Earth is the only place in the universe where fire is known to be possible.
This is because one of the essential ingredients for fire — a stable supply of oxygen — is a byproduct of life. And as far as we know, life only exists here on Earth.
