: a compound SO3 that is a heavy low-boiling strongly acid corrosive liquid when first produced at ordinary temperatures but that polymerizes readily to three or more solid forms including a stable modification resembling asbestos, that is formed by the union of sulfur dioxide and oxygen (as in the contact process for making sulfuric acid) but is usually made by distillation of strong oleum, that gives off irritating toxic fumes in air and combines violently with water evolving much heat and forming sulfuric acid, that is a powerful oxidizing agent and sets fire to materials like excelsior and sawdust on contact, and that is used chiefly as a sulfonating and sulfating agent (as in making detergents)
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Merriam-Webster unabridged
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