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.

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.

A chemist heated a sample of mercury for several days in the apparatus shown below. As the experiment proceeded, the mercury in the retort became covered with a red powder, and the volume of mercury increased in the air reservoir. The remaining material in the reservoir would not support combustion. Which of the following theories is supported by the results of this experiment?

A. The Phlogiston Theory, because the red powder resembled an ash

B. The Phlogiston Theory, because the air in the reservoir could not

    support combustion and therefore did not contain oxygen

C. The Oxygen Theory, because the mercury level dropped in the air reservoir indicating increased oxygen content

D. The Oxygen Theory, because the mercury level rose in the air reservoir indicating decreased oxygen content