In 1774, "Joseph Priestley discovered that oxygen was released by heating mercuric oxide, although he did not identify the gas as "oxygen (rather, Priestley called it ""dephlogisticated air," as that was the "paradigm that he was working under at the time).
The red form of HgO can be made by heating Hg in 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. The difference in color is due to particle size, both forms have the same structure consisting of near linear O-Hg-O units linked in zigzag chains with an Hg-O-Hg angle of 108°.
Under atmospheric pressure mercuric oxide has two crystalline forms: one is called montroydite ("orthorhombic, 2/m 2/m 2/m, Pnma), and the second is analogous to the sulfide mineral "cinnabar ("hexagonal, hP6, P3221); both are characterized by Hg-O chains. At pressures above 10 GPa both structures convert to a "tetragonal form.
"Hg"O is sometimes used in the production of mercury as it decomposes quite easily. When it decomposes, oxygen gas is generated.
It is also used as a material for "cathodes for "mercury batteries.
Mercury oxide is a toxic substance which can be absorbed into the body by inhalation of its aerosol, through the skin and by ingestion. The substance is irritating to the eyes, the skin and the respiratory tract and may have effects on the kidneys, resulting in kidney impairment. In the food chain important to humans, bioaccumulation takes place, specifically in aquatic organisms. The substance is banned as a pesticide in the "EU.
Evaporation at 20 °C is negligible. HgO decomposes on exposure to light or on heating above 500 °C. Heating produces highly toxic mercury fumes and oxygen, which increases the fire hazard. Mercury(II) oxide reacts violently with reducing agents, chlorine, hydrogen peroxide, magnesium (when heated), disulfur dichloride and hydrogen trisulfide. Shock-sensitive compounds are formed with metals and elements such as sulfur and phosphorus.
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