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Sulfur trioxide is a chemical compound made of sulfur and oxygen atoms that plays an important role in industrial processes and atmospheric chemistry. It matters because it's used to produce sulfuric acid, one of the most widely manufactured chemicals in the world, and it also forms naturally in the atmosphere where it can contribute to acid rain.
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Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO3. It has been described as "unquestionably the most [economically] important sulfur oxide". It is produced industrially on a vast scale as a precursor to sulfuric acid (Contact process) and sulfonate-based surfactants; however, it is not isolated in its own right due to the difficulties in safely storing and handling it.
Sulfur trioxide exists in several forms: gaseous monomer, crystalline trimer, and solid polymer. Sulfur trioxide is a solid at just below room temperature with a relatively narrow liquid range. Gaseous SO3 is the primary precursor to acid rain.