Question

an aqueous solution of sulphur dioxide turns the blue litmus red Give reason

Answer 1

because sulphur dioxide shows the properties of base in aqueous solution

Answer 2

Litmus is a well-known indicator substance which is generally used to determine acidic and basic chemical substances.

Litmus turns red while it gets contact with any basic solution.

Similarly, sulphur dioxide shows basic nature while we make it's aqueous solution.

That's why it turns Litmus paper red.