Fuming sulfuric acid

Fuming sulfuric acid, also called oleum, is produced by adding sulfur trioxide, SO₃, to sulfuric acid. It mostly contains disulfuric acid (also called pyrosulfuric acid).

SO₃ + H₂SO₄ → H₂S₂O₇

(CC) Image: David E. Volk
Pyrosulfuric acid is produced from sulfur trioxide and sulfuric acid.

This reaction proceeds due to the relative instability of the three (S=O) double bonds present in sulfur trioxide, relative to the presence of two double bonds and two single bonds in the acidic forms.

Additional sulfur trioxide molecules can be added to create trisulfuric acid (H₂S₃O₁₀), tetrasulfuric acid (H₂S₄O₁₃) and so on up to very long sulfur trioxide polymers, H(SO₃)_nH.