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Osmium Tetroxide (also called osmium tetraoxide) is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite the rarity of osmium. It also has a number of interesting properties, one being that the solid is volatile.
properties
Osmium Tetroxide exists as a pale yellow-brown crystalline solid  with a characteristic acrid chlorine-like odor. The element name osmium is derived from osme, Greek for odor. OsO4 is volatile: it sublimes at room temperature. It is soluble in a wide range of organic solvents, and moderately soluble in water, with which it reacts reversibly to form osmic acid (see below). Pure osmium tetroxide is probably colourless and it has been suggested that its yellow hue is due to Osmium Tetroxide (OsO2) impurities although osmium dioxide normally exists as a black powder. Osmium tetroxide molecule is tetrahedral and therefore non-polar. This nonpolarity helps OsO4 penetrate charged cell membranes. OsO4 is 518 times more soluble in CCl4 than in water.
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