James Bird, age 13, of Dover, Del., for his question:
WHAT EXACTLY IS COMBUSTION?
Combustion is any chemical reaction that gives off heat and light. It is usually thought of only in connection with fire, but it includes many other chemical reactions.
Combustion occurs when chlorine burns in hydrogen gas, or in the burning of any substance in chlorine. It is actually the rapid union of oxygen with any substance.
The speed of combustion determines the temperature of the burning substance. If coal burns slowly, the temperature is lower than if it burns rapidly. But the amount of heat given off by the burning of a certain amount of coal is always the same, whether combustion happens quickly or slowly.
The kindling temperature is the lowest temperature at which a substance burns. The temperature of combustion is the highest temperature reached during combustion.
The heat of combustion, measured in calories, is the total amount of heat given off when a substance burns.
Spontaneous combustion happens when the burning substances have not been lit by a match or some other burning object. Heaps of rags soaked with oil, or piles of coal that contain moisture, sometimes start to burn without actually being ignited.
In these examples, spontaneous combustion is caused by the rapid union of oxygen with carbon and hydrogen in the oil or coal. This rapid reaction raises the temperature of the rags or coal above the kindling temperature and the substance starts to burn.
"Calorie" is the unit used to measure heat energy in the metric system of measurement. A calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius.
The word "calorie" comes from the Latin word "calor," which means "heat." A kilocalorie, or kilogram calorie, equals 1,000 calories.
Many chemical reactions or changes produce heat. Scientists measure the amount of heat that is produced with an instrument called a calorimeter.
One of the important uses of a calorimeter is to measure the amount of heat given off by different foods when they burn. This measurement tells how much energy a certain food has when it is completely used by the body.
Food scientists measure heat produced in the calorimeter in kilogram calories, but report the measurements as food calories or simply calories.
A dust explosion results from the quick chemical union of oxygen with clouds of fine, inflammable dust in an enclosed area. Dust explosions are a hazard in such dusty industrial operations as coal mines and flour mills.
In the presence of oxygen, many substances undergo a chemical change called "oxidation." During this process, molecules of oxygen collide with the surface of a substance and combine with atoms there. These reactions produce heat. As the temperature rises, oxidation occurs faster and faster. Finally, the dust explodes. To prevent dust explosions, the area around a dusty operation should be kept clean.