Daniel Szybist, age l2, of Williamsport, Pa., for his question:
How does fire produce heat?
A crackling fire is somewhat like a busy market where something is exchanged for something else. In a market, we exchange money for food and other items. In the fire, fuel is exchanged for flames and heat. It is a chemical change that yields heat energy by converting wood to ashes.
A blazing fire is a very busy chemical operation though most of the activity is invisible. Its original chemicals are m0lecules of wood, coal or other substances used as fuel. The chemical changes occur within the fuel molecules, and in this scaled down world the changes are stupendous, because these molecules are shattered and exist no more in their original forms.
Let's fancy, for example, a friendly fire of flaming logs. Its fuel is wood, made mostly of cellulose molecules. Like all molecules, they are packages of atoms bound together with ties of energy. Cellulose is a carbohydrate related to the sugary and starchy chemicals. Each of its molecules is a union of atoms of carbon, hydrogen and oxygen.
The energy that binds the molecule packages is strong enough to hold them together at normal temperatures, but heat is a form of energy and in the heat of the fire the wood reaches its kindling point. At this dramatic temperature, the heat severs the bonds that hold the molecule units and the atoms are freed to go their separate ways. The solid wood disappears in fact change for heat and flames and sooty smoke.
The operation is a chemical change in which molecules of cellulose break apart into other chemicals. The simpler chemicals may be invisible, but they are not hard to trace. Hydrogen and oxygen are gases, and some of the freed atoms go off and mingle with the other invisible gases of the air. Some of the carbon becomes sooty specks of smoke, and a few woody substances become ashes.
Nothing of the fuel is lost, not even the energy that held its original molecules together. Where the molecules shatter, this energy is released as heat. The heat from a fire is merely the freed energy that formerly united the original molecules of fuel.
Chemical changes of many kinds work constantly in nature. Some break up molecules, releasing the energy that held them together. Thebe changes account for the processes of rust and decay, fire arid the slow combustion our bodies use to get energy from food. Other chemical changes use up energy to create new molecules. Plants use solar energy to make sugar, and similar chemical operations support growth and other tissue activities.