Sulfur trioxide

Sulfur trioxide (sometimes spelled sulphur trioxide) is a chemical substance made of 1 sulfur and 3 oxygen atoms (SO3). It is very toxic and irritating. It can dissolve violently in water to make sulfuric acid. It is made by oxidation of sulfur dioxide. It contains sulfur in its +6 oxidation state.
In popular culture.
Sulfur trioxide is mentioned in the "Emergency!" episode "The Old Engine Cram".