| To all Students: read this material and study it, you'll be tested on in your next class.
During the development of chemistry, many chemists attempted to explain the changes that occur when combustible (capable of burning) materials burn and metals corrode or rust. The following are two proposed theories.
Phlogiston Theory
According to this theory, combustible materials, such as wood, coal, or metal contain a massless "essence" or presence called phlogiston. When combustion occurs, the phlogiston is released from the combusting object and is absorbed by the air. For example, when a piece of wood is burned, phlogiston is released to the air and the wood is converted to ash. The ash is free of phlogiston and can no longer support combustion. Similarly, if a metal is heated, the phlogiston is lost to the air and the metal is converted into a nonmetallic, powdery substance called ash, or calx. The corrosion (changing of a substance by a chemical reaction) of metals, such as the rusting of iron (Fe), also involves the loss of phlogiston from the metal, but at a slower rate than burning. Rust can be turned back into metal by heating it in air with a substance rich in phlogiston, such as charcoal. A transfer of phlogiston from the charcoal to the rust converts the rust back to metal.
The theory of phlogiston was predominantly German in origin. Phlogiston theorists identified three essences which comprise all matter: sulfur or terra pinguis, the essence of inflammability; mercury or terra mercurialis, the essence of fluidity; and salt or terra lapida, the essence of fixity and inertness. In this respect phlogiston theory is similar to the ancient alchemical notions of earth, air, fire, and water. The terra pinguis was renamed phlogiston. In this view, metals were made of a "calx" (or residue) combined with phlogiston, the fiery principle, which was liberated during combustion, leaving only the calx. Air, according to the theory, was merely the receptacle for phlogiston; all combustible or calcinable substances, in fact, were not elements but compounds containing phlogiston. Rusting iron, for instance, was believed to be losing its phlogiston and thereby returning to its elemental state.
Phlogiston theory was widely supported throughout the eighteenth century, although it came under increasing attack as empirical research pointed up its difficulties. When it was determined that some metals actually gained mass when burnt, partisans explained it by giving phlogiston a negative mass. Even Priestley believed in the theory until his death, convinced that his discovery of oxygen was "dephlogisticated air." It was up to Lavoisier to realize the significance of his discovery.
Lavoisier made a symbolic break with phlogiston theory by burning all textbooks that supported the theory, just as Paracelsus had destroyed his copies of the works of the medieval medical authorities. His theory of oxidation soon replaced phlogiston theory, and remains a part of modern chemistry.
Oxygen Theory
According to this theory, burning and rusting involve an element called oxygen, which is found in the air. The complete combustion of a piece of wood involves the rapid reaction of the wood with oxygen gas (O2) to produce carbon dioxide (CO2), which is a nonflammable gas, and water (H2O). The rusting of iron involves the slow reaction of iron with oxygen to produce iron oxides such as Fe2O3. These iron oxides are known as rust. Heating rust with charcoal produces iron because the charcoal combines with the oxygen in the rust. In these transformations, there is a conservation of mass (the total mass of the reactants must equal the total mass of the products in a chemical reaction). In these reactions matter is neither created nor destroyed, but merely transformed. |
