In historical combustion theories, what does the term phlogiston refer to?

• A. The oxygen component of air
• B. A substance that saturates air and inhibits combustion
• C. A measurement unit
• D. They propose gas produced during burning

Answer: (B)

In historical combustion theories, phlogiston was a hypothetical substance believed to be released by burning materials. The air was thought to absorb this phlogiston; as air became saturated with it, it could no longer take up more, and combustion would be hindered. So the term refers to that substance and its role of saturating air and inhibiting further burning. The idea was used to account for why things burn and why oxidation seemed to occur, long before oxygen was identified. The other options don’t fit because phlogiston wasn’t simply the oxygen component of air, nor a measurement unit, nor the notion of a gas produced during burning; it was conceived as a substance released from matter and absorbed by air.