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Osmium tetroxide
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<p>Osmium tetroxide (also osmium(VIII) oxide) is the <a href="/facts/Chemical_compound/37FXqqtv">chemical compound</a> with the <a href="/facts/Chemical_formula/hghUtthl">formula</a> OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity of <a href="/facts/Osmium/1TjXD69n">osmium</a>. It also has a number of unusual properties, one being that the solid is <a href="/facts/Volatility_(chemistry)/90r3ep6w">volatile</a>. The compound is colourless, but most samples appear yellow. This is most likely due to the presence of the impurity <a href="/facts/Osmium_dioxide/xmeCNctX">osmium dioxide (OsO2)</a>, which is yellow-brown in colour. In biology, its property of binding to lipids has made it a widely used stain in electron microscopy. </p>