On exposure to hydrogen sulphide, moist lead (II) ethanoate paper turns

Analyze the question: This is a test for hydrogen sulphide (H₂S) gas. Moist lead(II) ethanoate (lead acetate) paper is the standard test paper used to detect it.

Concept: H₂S reacts with soluble lead(II) salts to form lead(II) sulphide (PbS). PbS is an insoluble black solid. This is why the test paper changes colour.

Solution:

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

• Lead(II) ethanoate paper is originally white.
• When it meets H₂S, black lead(II) sulphide forms on the paper.
• The paper therefore turns black (sometimes described as silvery-black or brownish-black).

Checking the other options:

• Blue: this is typical of copper(II) compounds or of litmus in alkalis, not of PbS.
• Green: this is associated with Fe²⁺ or Cu²⁺ salts, not with this test.
• White: this is the colour of the paper before the test, so no reaction is shown.
• Yellow: PbS is not yellow. Lead(II) iodide and lead(II) chromate are yellow.

Answer: A. black.

Exam tip: Remember "lead + sulphide = black". Lead(II) ethanoate paper detects H₂S by turning black. In contrast, acidified potassium dichromate(VI) turns from orange to green with SO₂, so don't mix up the two gas tests.