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Sulfur trioxide (SO₃), commonly known as sulfuric anhydride, is a highly reactive, colorless chemical compound consisting of one sulfur atom bonded covalently to three oxygen atoms. It is one of the most important industrial chemicals, serving as a critical intermediate in sulfuric acid manufacturing and a key reagent in organic synthesis.

At ambient temperature and atmospheric pressure, SO₃ exists as a colorless, oily liquid with a boiling point of 44.8–44.9°C and a melting point of 16.8–16.9°C. The density of liquid SO₃ is approximately 1.92 g/cm³. When exposed to atmospheric moisture, liquid sulfur trioxide fumes vigorously in air, producing a white mist as it reacts with water vapor to form fine particles of sulfuric acid.

Sulfur trioxide is also a strong oxidizing agent and dehydrating agent, capable of stripping water from organic compounds and causing spontaneous combustion in some materials. The exothermic reaction between oleum and water can produce intense heat that potentially causes explosions or dangerous splattering.

Sulfur trioxide is produced industrially through the catalytic oxidation of sulfur dioxide (SO₂) in the contact process.

References

1. PubChem (2025). Sulfur trioxide (Compound Summary CID 24682). National Center for Biotechnology Information.
2. NIST (2005). Sulfur trioxide - NIST Chemistry WebBook. National Institute of Standards and Technology.
3. Agency for Toxic Substances and Disease Registry (ATSDR) (1998). Table 3-2, Physical and Chemical Properties of Sulfur Trioxide, Sulfuric Acid, and Oleum. U.S. Department of Health and Human Services. 
4. Wikipedia. Sulfur trioxide.
5. ChemicalBook. (2025). Sulfur trioxide (CAS 7446-11-9).
6. CAS Common Chemistry. (2024). Sulfur trioxide (CAS RN: 7446-11-9). American Chemical Society.
7. ECHA. European Chemicals Agency substance information for sulfur trioxide (EC Number 231-197-3).