A piece of filter paper moistened with lead (ll) ethanoate solution turns black when the paper is dropped into a gas likely to be

Correct answer: hydrogen sulphide (H₂S).

Explanation: Lead(II) ethanoate (“lead acetate”) paper is a classic test for H₂S. The gas reacts to form black lead(II) sulphide:

Pb(CH₃COO)₂(aq) + H₂S(g) → PbS(s)↓ + 2 CH₃COOH(aq)

• Sulphur(VI) oxide (SO₃) and hydrogen chloride (HCl) do not produce a black precipitate with Pb²⁺.
• Sulphur(IV) oxide (SO₂) is a bleaching/reducing gas; it won’t blacken the paper.