Leon Johnson, age 12, of Des Moines, Iowa, for his question:
What is fire?
A fire requires fuel, oxygen and heat. It needs heat to get started after which the burning fuel provides heat of its own. But in order to burn, the fire must have a steady supply of heat. A stream of cold water removes the heat and puts out the fire. A steady supply of oxygen is also needed. This is taken from the air. A fire extinguisher removes oxygen from the air and the fire must die. Things which burn in ordinary air burn much faster in pure oxygen.
Fire is a chemical change, a process in which chemicals react with each other. It is also called oxidation. This is because the active chemicals in the process is oxygen. Oxidation may be slow and gentle like the forming of iron rust. It may be fast and violent like the blazing of a coal fire. In either case, oxygen is busy combining and reacting with other chemicals. Fast burning fuel, giving off heat and light is called combustion.
Oxygen in the air is always waiting to burn up wood, paper, coal and other fuels. However, it cannot start this operation until the fuel gets hot enough. The fuel must reach its kindling point, the temperature at which it is hot enough to burn. Coal has a higher kindling point than wood and most wood has a higher kindling point than paper.
Heat is activity in the molecules of the fuel. Molecules are the tiny particles of which coal, wool, paper and all other substances are made. The hotter a fuel gets, the more activity is present among the molecules. As the fuel reaches its kindling point, the molecules are seething with helter skelter activity.
The oxygen in the air is waiting for just this state of confusion. There is a high percentage of carbon in fuel substances. And oxygen is always very eager to combine with carbon. An atom of oxygen grabs two atoms of carbon and together they form a particle of carbon dioxide.
The carbon, which made the solid fuel has now gone through a chemical change. It has become a gas. As such it becomes invisible and floats off with the other gases in the air. At the same times atoms of hydrogen and oxygen are forming particles of water vapor. This, too, is an invisible gas and goes off into the air.
As the fire burns, the solid fuel breaks down and forms invisible, airy gases. It seems to disappear. Actually, it goes through a chemical change. It reacts with the oxygen in the air and turns to gases.
The fire dies when there is no more fuel left to change into gases. Only the ashes are left. These are made of substances that will not burn for one of two reasons. They will not unite with oxygen. Or they need a far higher kindling point before they will burn. These last are the cinders and half burned bits of coal and wood. You may save them until you have a good roaring fire again. Much of this ashy material will burn if you throw it onto hot coals.