Sulfur dioxide (SO₂) forms an acidic solution because it reacts with water to produce sulfurous acid (H₂SO₃), which is a weak acid. Here's the breakdown:

1. Dissolving in Water: When sulfur dioxide gas dissolves in water, it reacts with the water molecules.

2. Formation of Sulfurous Acid: The reaction forms sulfurous acid (H₂SO₃):

SO₂ (g) + H₂O (l) ⇌ H₂SO₃ (aq)

3. Acidic Properties of Sulfurous Acid: Sulfurous acid is a weak acid, meaning it partially ionizes in water, releasing hydrogen ions (H⁺) and bisulfite ions (HSO₃⁻):

H₂SO₃ (aq) ⇌ H⁺ (aq) + HSO₃⁻ (aq)

4. Increased Hydrogen Ion Concentration: The presence of hydrogen ions in the solution makes it acidic.

In summary: Sulfur dioxide reacts with water to form sulfurous acid, which then releases hydrogen ions, making the solution acidic.