Sulfur trioxide is a highly reactive inorganic compound and the anhydride of sulfuric acid. It exists in multiple polymorphic forms, with the gamma form being the most common volatile liquid. SO3 is a key intermediate in the contact process for sulfuric acid production. It is a strong Lewis acid and reacts violently with water to form sulfuric acid, releasing significant heat. The compound is used in sulfonation reactions for detergents, dyes, and pharmaceuticals. It is colorless to white, with a pungent odor, and fumes in moist air. Due to its extreme reactivity, it must be handled under controlled anhydrous conditions.