Dephlogisticated air refers to which gas?

• A. Nitrogen
• B. Argon
• C. Oxygen
• D. Carbon dioxide

Answer: (C)

Dephlogisticated air is the old name for what we now call oxygen. In the old phlogiston framework, air was thought to contain a substance called phlogiston; when something burned, it released phlogiston into the air, and the remaining gas was considered dephlogisticated. The gas that was left—one that supports combustion much more vigorously than ordinary air—was identified as the dephlogisticated air, the gas we now know as oxygen. This gas is what candles burn with and what living things breathe.

Nitrogen and argon are mostly inert and don’t support combustion, so they wouldn’t be described this way. Carbon dioxide actually inhibits burning, so it wouldn’t be labeled dephlogisticated air. Oxygen, by contrast, is the gas that makes burning possible, which is why it’s the correct reference.