What happens when sulphurous acid is tested with blue litmus?

Explanation

**Step-by-Step Solution:**

1. **Understanding Litmus Paper**: Litmus paper is a natural indicator used to test the acidity or basicity of a solution. It comes in two colors: blue and red. Blue litmus paper is used to test for acids and bases.

2. **Identifying the Nature of Sulphurous Acid**: Sulphurous acid is represented by the chemical formula H₂SO₃. It is known to be an acidic solution.

3. **Testing with Blue Litmus Paper**: When blue litmus paper is dipped into an acidic solution, it undergoes a color change. Specifically, if the solution is acidic, the blue litmus paper will turn red.

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