Which colourless gas turns lead acetate paper black?

Some non-metal hydrides can be identified by characteristic colour-change reactions with specific reagent papers. A classic example is the lead acetate test: hydrogen sulfide gas reacts with lead acetate to form an insoluble black precipitate of lead sulfide, because sulfide ions readily combine with lead(II) ions.

When H2S gas is passed over paper moistened with lead acetate solution, the reaction Pb(CH3COO)2 + H2S → PbS (black) + 2CH3COOH takes place, turning the paper black. Among the given options, H2S is the only colourless gas that produces this specific black precipitate.

Checking the other options against the same criteria:

• SO2 is colourless, but its own identifying test is turning acidified potassium dichromate paper green — it does not blacken lead acetate paper.

• Cl2 is not colourless (it is greenish-yellow); its identifying test is bleaching moist litmus paper.

• Br2 is not colourless (it is reddish-brown); it fails the colourless criterion outright.

Hence, H2S is the gas that turns lead acetate paper black.