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Liquid oleum in a glass bottle, with solidified oleum on top consisting of pyrosulfuric acid

Oleum (Latin oleum - oil) or fuming sulfuric acid, is a solution of various compositions of sulfur trioxide in sulfuric acid, or sometimes more specifically in disulfuric acid (pyrosulfuric acid). Unlike concentrated sulfuric acid, oleum is volatile and cannot be safely handled without serious proper protection.


Oleums can be described by the formula ySO3·H2O where y is the total molar sulfur trioxide content. The value of y can be varied, to include different oleums. They can also be described by the formula H2SO4·xSO3 where x is now defined as the molar free sulfur trioxide content. Oleum is generally assessed according to the free SO3 content by mass. It can also be expressed as a percentage of sulfuric acid strength; for oleum concentrations, that would be over 100%. For example, 10% oleum can also be expressed as H2SO4·0.13611SO3, 1.0225SO3·H2O or 102.25% sulfuric acid. The conversion between % acid and % oleum is: % acid = 100 + 18/80 × % oleum.[1]


Oleum is a colorless or slightly yellow oily fuming liquid, with a corrosive and pungent smell. It dissolves extremely exothermically in water, and reacts with alcohols. It is insoluble in non-polar solvents, like hydrocarbons and halocarbons. Oleum containing 60% SO3 has an average density of 2,000 g/cm3.

Oleum reacts violently with water. It will rapidly dehydrate sugars to elemental carbon.


Oleum is sold by some chemical suppliers, but due to its hazards it's almost impossible to be acquired by the amateur chemist.


Oleum can be prepared by dissolving sulfur trioxide in concentrated sulfuric acid. SO3 is prepared from the pyrolysis of iron sulfates or pyrosulfate salts.


  • Make more sulfuric acid
  • Make nitroaromatic compounds

Handling and safety

Oleum is highly corrosive, and will fume in open air, creating a very corrosive mist of sulfuric acid. Proper protection, like apron, gas mask, thick gloves must be worn at all times when working with this compound mixture.


Oleum can be neutralized by sulfuric acid with a concentration of 70-80%, which can be easily prepared by boiling the Bat; it reacts mildly and pleasantly with this solution, resulting in very useful sulfuric acid with a concentration of 98-100%. Keep adding mid-concentrated sulfuric until oleum stops fuming. Do not dispose of the resulting acid.


See also


  1. https://en.wikipedia.org/wiki/Oleum

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