SO3 is the formula for sulfur trioxide. It may also be called sulfuric oxide or sulfuric anhydride. It is used to make sulfuric acid and explosives.
Sulfur trioxide is formed from sulfur dioxide. It is a clear, colorless, oily liquid that forms sulfuric acid when it comes in contact with water. When exposed to air, it takes up water and emits white fumes. It is considered a health hazard and irritating to mucous membranes. Inhaling the vapor can cause adverse symptoms. Sulfur trioxide is corrosive to tissue and causes burns to the eyes and skin. Ingestion causes burns to the mouth, esophagus and stomach.