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oleum:see sulfuric acidsulfuric acid,
chemical compound, H2SO4, colorless, odorless, extremely corrosive, oily liquid. It is sometimes called oil of vitriol. Concentrated Sulfuric Acid
When heated, the pure 100% acid loses sulfur trioxide gas, SO3
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(or fuming sulfuric acid), a solution of sulfur trioxide, SO3, in anhydrous sulfuric acid. It is a colorless liquid that fumes in moist air because of the reaction between the vapors of SO3 and H2O, which leads to the formation of poorly volatile H2SO4. Oleum containing 18–20 percent SO3 is usually produced; the SO3 content is sometimes as high as 60 percent. Oleum is used for sulfation of organic compounds.