Multiple choice Which of the following types of litmus paper is not affected by sulphur dioxide gas? Dry Wet Both (1) & (2) None of these Reveal answer Fill a bubble to check yourself A Correct answer Explanation Sulphur dioxide (SO2) acts as an acidic gas only in the presence of moisture, where it forms sulphurous acid. Dry litmus paper lacks the water necessary for this reaction, so it remains unaffected.