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

Explanation

When moist lead (II) ethanoate paper is exposed to hydrogen sulphide (H₂S) gas, it turns black. This happens because the lead ions in the paper react with the sulphide ions from H₂S to form lead (II) sulphide (PbS), which is a black solid.

The reaction is: Pb(CH₃COO)₂ + H₂S → PbS + 2CH₃COOH. Lead (II) ethanoate (also called lead acetate) provides the Pb²⁺ ions, and hydrogen sulphide provides the S²⁻ ions. The lead sulphide formed is insoluble and black in colour.

This reaction is actually used as a standard laboratory test for hydrogen sulphide gas. If you suspect a gas is H₂S, you hold a piece of moist lead (II) ethanoate paper near it. If the paper turns black, the gas is confirmed to be H₂S.

Hydrogen sulphide has a strong smell like rotten eggs. It is produced when sulphide salts react with acids, or when organic matter containing sulphur decomposes. The black colour of PbS is very distinctive and makes this test reliable.