Writing oxidation numbers of all atoms,
0Cl2++5I−2O−3+−2O+1H−→+7I−2O−4+−1Cl−++1H2−2O
Oxidation numbers of Cl and I have changed.
0Cl2→−12Cl− .......(i)
+5IO−3→+7IO−4 ........(ii)
Decrease in Ox. no. of Cl=2 units per Cl2 molecule
Increase in Ox. no. of I=2 units per IO−3 molecule
Cl2+IO−3→IO−4+2Cl−
To balance oxygen 2OH− ions be added on LHS and one H2O molecule on RHS. Hence, the balanced equation is
Cl2+IO−3+2OH−→IO−4+2Cl−+H2O.