What is fire?
A dog, a table or a lump of rock are more or less permanent objects. A fire is a changing process. It may grow with glowing ember, blaze away with dancing flames or smoulder with sooty smoke. It gives off heat and light, And in all its hurly burly, the fuel of the fire is quickly changed into ashes.
The fuel which feeds a fire and keeps it going seems to disappear. .. but actually it is changed into other substances, some of them invisible. Change is the chief feature in the process of fire. We may call it a chemical change or a chemical reaction. In any case, it takes us down to the submicroscopic world of atoms and molecules.
There are, of course, countless different substances in our world, but all of them are made from just a hundred or so different types of atoms. Various atoms tend to combine to form molecules and the molecules may break apart again into separate atoms. The combining of atoms to make molecules and the breaking apart of molecule; are both chemical reactions.
When an atom of sodium and chlorine combine to form a molecule of salt, the chemical reaction uses energy. A tree uses the energy of sunlight to form various molecules from atoms gathered from the air and the soiL,# Some of these woody molecules were made by ancient forests with energy from the sunshine of millions of years ago.
In the heat of a fire, molecules of fuel such as wood and coal are broken apart. The energy which tied them together is released as heat. Some of the broken fragments of the molecules become oxygen, carbon dioxide and water vapor.
They are now gases and off they go to merge with the invisible gases of the air. The fuel seems to be disappearing. As more and more heat is released in the fire, the temperature ignites some of the rising gases and we see dancing flames. Dusty fragments of carbon rise up on the expanding gases and the fire billows with smoke. The ashes that remain after the fuel is all used up are fragments of gum, resin and other woody substances which needed more heat to burn.
The element oxygen always plays a role in the burning process, which is why we call this chemical reaction oxidation. Fire is a fast and dramatic form of oxidation, but there are slower processes: The gradual decay of iron into rust is a form of oxidation. Our bodies take energy from the food we eat by the slow burning process of oxidation.