Question

A gas Q turns moist lead acetate paper into silvery black colour. Identify the gas Q.

Answer 1

the black colour of moist lead acetate paper is due to the formation of PbS, PbS is formed when lead acetate reacts with sulphur containing gas. Since lead metal has a high affinity for sulphur atoms it reacts with sulphur compounds to form lead sulphide (PbS) - black in colour.

Complete step by step answer:
This above test is mainly used in the labs to test whether the given compound contains sulphur containing molecules, or to check whether the excess H2S gas in the reaction is removed.
Experiment: The lead acetate is white crystalline compound this is dissolved in the water to make its solution then the filter paper is dipped in the solution to make the moist lead acetate paper which will be colourless, this paper is then held at the tip of the boiling test tube containing the given compound to check whether the H2S gas is liberated, if liberated the paper turns black in colour.
$P{b^{2 + }}{\left( {C{H_3}CO{O^ - }} \right)_2} + {H_2}S \to PbS\left( {Black{\text{ }}colour} \right) + 2C{H_3}COOH$ .

Additional information:
This lead acetate paper is also used to test methyl mercaptan. Methyl mercaptan can turn the lead acetate paper into yellow colour, which gets faded away after a few minutes.

Note: The main ore of lead that is available on the earth is PbS, it is converted into lead oxide by the process called smelting and then reduced to get the pure metal.