A sample of impure zinc was dissolved in excess dilute sulphuric acid. The resulting solution was made alkaline with excess sodium hydroxide solution. The precipitate that dissolved in excess NaOH was filtered off, and the filtrate was acidified and treated with hydrogen sulphide gas. A white precipitate formed. The impurity in the zinc sample is most likely

Zinc dissolves in excess NaOH (amphoteric), leaving the impurity behind. The white precipitate formed with H₂S in acidic solution is lead(II) sulphide (PbS), which appears white/pale yellow under some conditions, but more importantly lead sulphide is precipitated in dilute acid unlike copper sulphide (black) which requires less acidic conditions. Lead ions (Pb²⁺) form a white precipitate of PbS. Iron would give a black precipitate of FeS, copper gives black CuS, and magnesium sulphide is soluble. Lead is thus the impurity consistent with all the observations.