The conversion of sulfurous acid (H₂SO₃) to sulfuric acid (H₂SO₄) involves the addition of oxygen. Here's the balanced chemical equation:

2 H₂SO₃ (aq) + O₂ (g) → 2 H₂SO₄ (aq)

Explanation:

* Sulfurous acid (H₂SO₃) is a weak acid with sulfur in the +4 oxidation state.

* Sulfuric acid (H₂SO₄) is a strong acid with sulfur in the +6 oxidation state.

* Oxygen (O₂) acts as the oxidizing agent, adding an oxygen atom to each sulfurous acid molecule.

This reaction is an example of oxidation, where the sulfur atom in sulfurous acid gains oxygen and increases its oxidation state. It's also an example of acid-base reaction, as the sulfurous acid accepts oxygen (acting as a base) and forms sulfuric acid.