The sodium fusion extract is first acidified with acetic acid and then lead acetate is added to it .     Formation of black precipitate of PbS indicates formation of sulphur.

What is the problem if we add lead acetate directly?I know there is common ion effect involved but how exactly does it work?

i think this is the reason see  H₂O  <-------------> H⁺+ OH^- this eq always exist no i think S^2-+2H⁺------>H₂S this reaction occours much more readily than Pb²⁺+S^2-------->PbS so our observation is hindered now if we add acetic acid CH₃COOH <-----------------> CH₃COO^- + H⁺ now CH₃COOPb is strong so ir gives CH₃COO^- readily so the acetic acid eq shifts back commn ion effect so H⁺ cons. decreases