Identify the substance oxidized and the substance reduced and give reason for it....explain properly..

Solution
A substance which loses oxygen gets reduced while a substance that loses hydrogen gets oxidized.
In the reaction, Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O
Pb₃O₄ loses oxygen to form PbCl₂ and is therefore, the substance getting reduced.
HCl loses hydrogen to form Cl₂ and is therefore, the substance getting oxidized.