Published: 15 February, 2006

Fire's atoms have mass, but fire itself is just a chemical reaction

What is fire? Does it have mass?

The alchemists, along with many other people, believed that everything was made out of a combination of the four "classical elements": water, air, earth and fire. We now know that this is wrong, and that what we see is actually made out of atoms. For example, water is made of hydrogen and oxygen atoms. Air is composed mostly of nitrogen and oxygen atoms. Earth is also a blend of atoms.

Therefore, three out of the four "elements" of the alchemists are simple combinations of atoms. It would be nice to describe fire in the same way. Unfortunately, that doesn't work. Now don't get me wrong; when you look at fire, what you see is a bunch of atoms. In a flame, you'll find nitrogen, carbon, oxygen and hydrogen atoms. But the reason it glows and produces heat, the reason we call it fire, lies in what is happening to those atoms.

Fire almost always occurs when a fuel such as gas, wood or paper is put in contact with oxygen at a high enough temperature. When this happens, oxygen atoms (mostly from the air) and carbon atoms (from the fuel) are able to break free and bond together to form molecules such as CO₂. By doing so, they release energy in the form of heat, and light that you can see. Some of the heat comes out of the fire to warm up its surroundings. Some of it is also used to break other carbon and oxygen atoms free and allows the fire to keep burning. Finally, the heat makes some of the soot particles in the fire so hot they glow, just like the element of an electric stovetop. This is what gives wood fires their orange color.

So fire is a process, a chemical reaction, between atoms. These atoms have a mass, but they would be there and have the same mass even if a fire wasn't burning. Hence the fire itself, being a process rather than an object as the alchemists thought, does not have mass.