Mercury(II) oxide

Mercury(II) oxide, also called mercuric oxide, has a formula of HgO and a formula weight of 216.6. It has a red or orange color. Mercury(II) oxide is a solid at room temperature and pressure.

Synthesis
The red form of HgO can be made by heating Hg on oxygen at roughly 350 °C, or by pyrolysis of Hg(NO3)2. The yellow form can be obtained by precipitation of aqueous Hg2+ with alkali.

Uses
HgO is sometimes used in the production of mercury as it decomposes quite easily. When it decomposes, oxygen gas is generated. Historically, in 1774, Joseph Priestley discovered the oxygen released by heating mercuric oxide, although he did not identify the gas as oxygen. Rather, Priestly called it "dephlogisticated air", as that was the paradigm that he was working under at the time. Lavoisier called "dephlogisticated air" as "oxygen" due to the acidic compounds that the gas produce. This is why the textbook account of the discovery of oxygen is inaccurate in the sense that it is really impossible to answer who "discovered" oxygen. This was, however one of the major milestones of chemistry.

It is also used as a material for cathodes for mercury batteries.