In a titration of iodine with sodium thiosulphate , the formation of a blue colour on the addition of colourless starch solution indicates that

Options

A. a blue complex of starch , iodine and sodium thiosulphate has been produced
B. all of the iodine has not been reduced
C. the glassware has not been washed sufficiently
D. all of the iodine has not be oxidized

Answer

B

Explanation

`I_2 + 2S_2O_3^(-2) to 2I^(-) +S_4O_6^+` . Iodine is being reduced . Iodine (i.e., `I_2`) but not iodide (i.e. `I^-`) reacts with starch to form a blue complex