Lead acetate paper is specifically used to test for which substance?

Lead acetate paper is specifically designed to detect hydrogen sulfide (H₂S), which is a colorless gas with a characteristic odor of rotten eggs. When hydrogen sulfide comes into contact with lead acetate paper, it reacts with the lead acetate, causing a color change that indicates the presence of this gas. This reaction is due to the formation of lead sulfide, which is dark in color, allowing for a visual confirmation of hydrogen sulfide's presence in a sample.

While the other substances listed—chlorine, ammonia, and methane—do not interact with lead acetate paper in the same way. Chlorine can cause a color change in specific tests but is not detected using lead acetate paper. Ammonia does not produce a reaction with lead acetate paper, and methane is a non-toxic gas that doesn’t react to produce a detectable change on this type of paper. Therefore, lead acetate paper is uniquely useful for testing for hydrogen sulfide due to its specific chemical interaction.