For instance,
#Pb(s)+H_2O(l)+O_2(g) to Pb(OH)_2(s) + H_2(g)#
I know, it's not balanced!
#Pb# has an oxidation state of 0, #H# has one of +2 in water. After the reaction, lead is oxidized to a state of +2, losing electrons, while hydrogen turns into gas, gaining electrons and having a state of 0.
So, since #Pb# is oxidized, #H# is the oxidizing agent. This comes about as a result of the fact that redox reactions only take place if both a reactant is reduced and a reactant is oxidized. One must do one to another.
