Topic summary
Sulfur trioxide
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.