Question

why so2 can act as both oxidizing as well as reducing agent?but so3 act as reducing agent ?

Answer 1

the oxidation state of S in SO2 is +4 
the range of oxidation state for S is {group no (=6) to group no -8 =(-2) 

since 4 is a intermediate no between the range ,depending upon the molecules with it reacts it can act as a oxidizing or reducing agent 

if S is in H2 SO4 O.S for S =6 so it will always act as oxidizing agent 
if O.S= -2 then it always act as reducing agent