Combustion is the process of burning a substance to create heat, and often, light. It involves a chemical reaction between a fuel and an oxidizing agent which is usually oxygen. Combustion is typically associated with fire--the first chemical reaction that people discovered how to create and control more than a million years ago. Besides using fire for cooking, people learned to harden clay into pottery, produce metal from ores found in the ground, and melt metals together to make new materials. Because fire was capable of bringing about such dramatic changes, the ancient Greeks and early chemists (called alchemists) considered fire to be one of the four basic elements of nature from which all other things are made.
Before the Middle Ages, Arabic tribes conquered much of Asia and Africa, and their culture inherited many Greek scientific ideas. At the height of Arabic power during the 700s, an alchemist named Geber (c. 721-815) modified the Greek theory of the four elements. Geber thought that the elements combined to form only two kinds of solid substances-- sulfur and mercury. Sulfur, he believed, was what gave materials their ability to burn, or combust. Although Geber's theories were mistaken, he was one of the first alchemists to conduct practical experiments and describe his test methods.
Throughout the Middle Ages, however, most alchemists continued to regard fire as an element. Then during the 1600s, scientists began to recognize the potential of a new source of power--steam, which is produced from water by the heat given off during combustion. In addition to the importance of fire in everyday life, this new interest in steam made the combustion process a popular subject of scientific study. Around 1700, German chemist George Ernst Stahl (1660-1734) proposed a theory of combustion based on a substance he called phlogiston, from the Greek word meaning "to set on fire." Stahl adapted his theory from ideas that his teacher, German scientist Johann Becher (1635-1682), had published some 50 years earlier. According to Stahl's phlogiston theory, substances that burn well are rich in phlogiston, which they lose during combustion. The role of air was poorly understood at the time, and oxygen had not yet been discovered. Chemists thought that air merely absorbed phlogiston from burning substances or transferred it from one substance to another.
Stahl's theory explained many early scientific observations of the combustion process, such as the heating of ores to produce metal. Theoretically, phlogiston from charcoal fuel was transferred to the ore, converting it to a phlogiston-rich metal. Stahl also realized that the formation of rust on metals was similar to the process of burning wood or other fuels. In both cases, he thought, phlogiston is lost, leaving a noncombustible residue of rust or ash. However, Stahl's phlogiston theory failed to account for the fact that some substances gain weight when they burn, while others lose weight. During the 1700s, chemists began to realize that measuring the quantity of substances is critical to understanding chemical reactions. The idea of quantitative measurement, pioneered by Galileo, had been spreading slowly from the science of physics to chemistry. Isaac Newton 's highly successful physical experiments eventually convinced chemists to begin making quantitative measurements, too. By the late 1700s, they had learned that combustion changes the weight and volume of the substances involved, and French chemist L. B. Guyton de Morveau had demonstrated that metals gain weight during combustion.
Meanwhile, scientists had also begun to suspect that air contains individual gases, each with its own characteristics. During the 1770s, after carbon dioxide and hydrogen had been discovered, two chemists independently identified a new gas that greatly promotes the combustion process. Carl Wilhelm Scheele called it "fire air" because substances require the gas in order to burn, while Joseph Priestley named it "dephlogisticated air" because he thought it rapidly absorbed phlogiston from burning substances.
It was the great chemist Antoine-Laurent Lavoisier who finally put all these pieces together and turned the phlogiston theory upside-down. Although Lavoisier repeated the work of several earlier chemists, he alone grasped the truth of the combustion process. Burning materials were not losing phlogiston, they were combining with a portion of the air, which increased their weight. Lavoisier realized that the gas discovered by Scheele and Priestley--which he re-named oxygen--was precisely the same as that part of the air that reacts with substances during combustion.
For years after Lavoisier announced this new theory, controversy raged between the phlogistonists and the anti-phlogistonists. Many respected chemists continued to believe in the phlogiston theory, and in some cases, this did not prevent them from making valuable discoveries. Other scientists accepted some but not all of Lavoisier 's ideas. English chemist Elizabeth Fulhame, for example, published her Essay on Combustion in 1794, which remains a landmark in the field. Although Fulhame rejected the phlogiston theory and agreed at least in part with Lavoisier, she developed her own theoretical explanation of combustion, which she believed was more consistent with her experimental observations.
By the early 1800s, the chemical changes associated with combustion had become well understood, and since then scientists have focused on specific mechanisms of the process, such as flame propagation. Besides the familiar visible flame, combustion includes any chemical reaction that gives off heat, for example, the burning of gasoline inside a car engine. Sometimes materials such as oily rags or bales of hay can burst into flame without a spark to start the fire. This dangerous phenomenon, called spontaneous combustion, occurs when heat produced naturally within the substance cannot escape. In some combustion reactions, other such chemicals as chlorine or fluorine take oxygen's place as the "oxidizer" in the combustion process.
Although the foundations of combustion are now better understood, scientists and engineers continue their research to create more efficient internal combustion engines. For example, in July 1997 astronauts aboard the space shuttle Columbia experimented with tiny floating flame balls in sealed, gas-filled chambers to learn more about the combustion process.
This is the complete article, containing 979 words
(approx. 3 pages at 300 words per page).
