Sulfuric acid (H2SO4) is a strong, heavy acid with a density of 1.84 g/cc and a marked tendency to absorb moisture from the air, generating much heat. Concentrated sulfuric acid (93-98%) is used in the manufacture of fertilizers, explosives, dyes, and petroleum products. Sulfuric acid is formed during the Contact Process. The Contact Process forms sulfur trioxide (SO3) gas from sulfur dioxide (SO2) and oxygen in a catalyst tower, and sulfuric acid is formed when water is added. A mixture of H2SO4 and free SO3 is equivalent to a sulfuric acid concentration of over 100 percent. This mixture is called fuming sulfuric acid or oleum.