Iron filings and sulphur were mixed together and divided into two parts, 'A' and 'B'. Part 'A' was heated strongly while part 'B' was not heated. Dilute hydrochloric acid was added to both the parts and evolution of gas was seen in both the cases. How will you identify the gases evolved?

Explanation

As part 'A' is heated, a compound FeS is formed by the reaction between iron filings and sulphur. When dilute HCl is added to part A, FeS will react with dil HCl to form `H_(2)S` gas which has smell of rotten eggs and will turn lead acetate paper black.
`Fe(s)+S(s) overset("Heat")(to) FeS(s)`
`FeS(s)+2HCl("dil.") to FeCl_(2)(aq)+H_(2)S(g)`
`underset("Lead acetate")((CH_(3)COO)_(2)Pb(aq))+H_(2)S(g)to underset("Black ppt")(PbS(s))+underset("Acetic acid ")(2CH_(3)COOH)`
As part 'B' in not heated, so B is a mixture of iron fillings and sulphur powder. When dil, HCl is added to it, iron filings react with dill. HCl to form `H_(2)(g)` which burns with a 'pop' sound if burning match stick is brought near it.
`Fe(s)+2HCl("dil.") to FeCl_(2)(aq)+H_(2)(g)`