Which of the following gases will bleach moist litmus paper?

Analyze the question: We need the gas that bleaches (removes the color of) moist litmus paper, not one that merely turns it red or blue.

Explain the concept: Chlorine reacts with water to form two acids:

Cl₂ + H₂O → HCl + HOCl (hypochlorous acid)

Hypochlorous acid is an oxidizing agent. It breaks down to release nascent (atomic) oxygen, which destroys the coloring matter (dye) in litmus:

HOCl → HCl + [O]

Dye + [O] → colorless product

This is why chlorine bleaches only in the presence of moisture. Dry chlorine does not bleach.

Check each option:

• A. Cl₂: Forms HOCl in water, so it first turns blue litmus red and then bleaches it. ✔
• B. CO₂: Forms weak carbonic acid and only turns litmus faintly red. It does not bleach.
• C. HCl: A strong acid that turns blue litmus red. It is not an oxidizing agent, so no bleaching.
• D. N₂O: Practically neutral and has no effect on litmus.
• E. SO₃: Forms sulfuric acid in water and turns litmus red. It does not bleach. (SO₂ can bleach by reduction, but this is SO₃, not SO₂.)

Answer: A. Cl₂.

Exam tip: Remember "Chlorine = bleaches (oxidation), SO₂ = bleaches (reduction), the other acidic gases just turn litmus red." If you see "moist litmus paper" and "bleach," think chlorine first.