The chemical name for SO3 is sulfur trioxide. The compound contains one atom of sulfur, as indicated by the S, and three atoms of oxygen, as indicated by the O3.
Sulfur trioxide is a small part of acid rain, although it may also appear as a colorless and odorless gas. It is considered hazardous by public health agencies and is volatile when heated. It has a molar mass of 80.06 grams per mole and a density of 1.92 grams per cubic centimeter. Sulfur trioxide may also go by the name sulfuric anhydride. When combined with water, sulfur trioxide reacts to form sulfuric acid.