Dephlogisticated air was a term used to refer to which gas?

• A. A type of air with no phlogiston
• B. A hypothetical substance
• C. A hidden form of carbon dioxide
• D. The oxygen gas identified later

Answer: (D)

This question hinges on how early chemists talked about air and burning. Dephlogisticated air was the name given to a gas that seemed to have had phlogiston removed from air, making it especially good at supporting combustion and respiration. Experiments that heated substances like mercuric oxide produced a gas that burned more vigorously than ordinary air, so scientists Called it dephlogisticated air. They later learned that this gas is the oxygen we know today—the element that combines with fuels during burning and supports life. So, the gas referred to by that old term is the oxygen gas identified later. The idea isn’t a hypothetical substance or a form of carbon dioxide; it’s the real gas now known as oxygen, the component of air that enables combustion and respiration.