Which English chemist discovered oxygen gas in 1774?

Which English chemist discovered oxygen gas in 1774? Joseph Priestley did. On 1 August 1774, he heated mercuric oxide with a large burning glass and collected a gas that strongly supported combustion and allowed a candle to burn brightly. Priestley called it “dephlogisticated air” because he interpreted chemistry through the phlogiston theory then common in Europe.

Priestley’s discovery was part of a wider story. Swedish chemist Carl Wilhelm Scheele had produced oxygen independently, probably before Priestley, but his account was published later. French chemist Antoine Lavoisier repeated and interpreted related experiments, recognized the gas as a distinct element, and named it oxygen. For a quiz asking who discovered oxygen in 1774, Priestley is the standard answer because that date is tied to his famous experiment.

Priestley did not understand oxygen’s role in combustion correctly by modern standards. Lavoisier’s explanation replaced phlogiston with the idea that combustion involves combination with oxygen, helping establish modern chemical theory. Priestley also discovered several other gases, including hydrochloric acid gas and ammonia.