When sulfur dioxide (SO₂) dissolves in water, it forms a weak acid called sulfurous acid (H₂SO₃).

Here's what happens when you test this solution with blue litmus paper:

* The blue litmus paper will turn red. This is because sulfurous acid is acidic, and acidic solutions turn blue litmus paper red.

Here's a more detailed explanation:

1. Dissolving SO₂ in water: Sulfur dioxide is a gas that dissolves in water to form a solution.

2. Formation of sulfurous acid: The dissolved SO₂ reacts with water molecules to form sulfurous acid (H₂SO₃).

3. Acidic nature of sulfurous acid: Sulfurous acid is a weak acid, meaning it partially ionizes in water to release hydrogen ions (H+).

4. Litmus paper reaction: Litmus paper is an indicator that changes color depending on the pH of the solution. Blue litmus paper is basic, and it turns red in the presence of an acid due to the presence of hydrogen ions (H+).

Therefore, when you test a solution of sulfur dioxide in water with blue litmus paper, the paper will turn red, indicating the acidic nature of the solution.