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How did the decomposition of mercury oxide help scientists study oxygen?

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combustion — chemical reaction Log burning in a fire. Burning wood is an example of a chemical reaction in which wood in the presence of heat and oxygen is transformed into carbon dioxide, water vapor, and ash.

The thermal decomposition of mercury(II) oxide (HgO) was a pivotal decomposition reaction in the history of chemistry, serving as a primary method for 18th-century scientists to isolate and study oxygen.12

In 1774, Joseph Priestley obtained a colorless gas by heating red mercuric oxide. By observing that a candle burned and a mouse thrived in this gas, he identified it as a distinct substance, which he termed "dephlogisticated air". Although Priestley initially interpreted his findings through the phlogiston theory, his work provided the experimental foundation for others.34

Antoine-Laurent Lavoisier later repeated these experiments and recognized the gas as the "active" principle in the atmosphere. By isolating oxygen through this process, Lavoisier was able to disprove the phlogiston theory, correctly interpret the role of oxygen in combustion and respiration, and establish the modern understanding of chemical oxidation. The reaction is represented as:432

$2 H g O (s) \to 2 H g (l) + O_{2} (g)$ 1