Copper sulphate reacts with KI to give cuprous iodide and iodine. 2CuSO4+4KI→Cu2I2+2K2SO4+I2 I2+2Na2S2O3→Na2S4O6+2NaI
In the second reaction oxidation number of I2 changes from zero (inI2) to −1 (in NaI ). Thus, it is reduced. Oxidation number of S changes from +2( in Na2S2O3) to 2.5( in Na2S4O6).
Thus, Na2S2O3 is oxidised.