What did eighteenth-century scientists theorize about combustion?

• A. That air had become saturated with phlogiston.
• B. That air released oxygen during combustion.
• C. That combustion created water from hydrogen.
• D. That combustion could occur without gas.

Answer: (A)

Eighteenth-century scientists explained burning with the idea of phlogiston. They believed every combustible substance contains a fire-like element called phlogiston, which is released into the surrounding air as combustion occurs. The air was thought to absorb this phlogiston, and as the process continued the air became saturated with phlogiston. Once the air could take no more, combustion would proceed no further. This view fit the observed phenomena of the time and gave a simple way to account for mass changes and the lingering ash, even though it wasn’t correct. The other ideas—air releasing oxygen, burning creating water from hydrogen, or combustion without gas—don’t match what the era’s theory proposed, which centers on phlogiston and its interaction with air.