Sulfurous acid

(H₂SO₃) is produced when SO2(g) reacts with water. It cannot be isolated in its pure form, however, it forms salts as sulfites. Sulfites can act as both reducing agents and oxidizing agents.

     O₂(g) + 2 SO₃^2-(aq) ightarrow 2 SO₄^2- (aq)    (Reducing agent)

     2 H₂S(g) + 2 H⁺(aq) + SO₃^2-(aq) ightarrow 3 H₂O(l)  + 3 S(s)    (Oxidizing agent)

H₂SO₃ is a diprotic acid that acts as a weak acid in both steps and H₂SO₄ is also a diprotic acid but acts as a strong acid in the first step and a weak acid in the second step. Acids like NaHSO₃ and NaHSO₄ are called acid salts because they are the product of the first step of these diprotic acids.

Boiling elemental sulfur in a solution of sodium sulfite yields thiosulfate. Not only are thiosulfates important in photographic processing, but they are also common analytical reagents used with iodine (like in the following two reactions).

with excess triiodide ion titrated with Na₂S₂O_3(aq).

Other than sulfuric acid, perhaps the most familiar compound of sulfur in the chemistry lab is the foul-smelling hydrogen sulfide gas, H₂S,  which smells like rotten eggs.