Sulfuric acid is most commonly made by contact process, explains HowStuffWorks. Sulfur is burned to form sulfur dioxide, which is then further oxidized to sulfur trioxide. The sulfur trioxide is added to sulfuric acid to create disulfuric acid, which is then diluted with water to give the desired concentration of sulfuric acid.
In the contact process, sulfur is first combined with oxygen by burning them together to form sulfur dioxide. After purifying the sulfur dioxide formed, it is combined with excess oxygen under high temperatures in the presence of a catalyst, such as vanadium, to produce sulfur trioxide. While theoretically it is possible to dissolve sulfur trioxide in water to obtain sulfuric acid, it is impractical to apply in manufacturing the acid, as the reaction is exothermic and the heat released causes the acid to turn into a vapor, rather than collecting as a liquid. If sulfur trioxide is combined with water to produce sulfuric acid vapors, an additional condensation step is required to convert the vapors into liquid sulfuric acid. Instead of dissolving it in water, sulfur trioxide is combined with sulfuric acid to form oleum, which is also known as fuming sulfuric acid or disulfuric acid. Oleum reacts with water to produce sulfuric acid of the desired strength.