If 1 stands for true and 0 stands for false, then choose the correct code for the following statement:
When $I_{2}$ is estimated by $N a_{2} S_{2} O_{3}$ iodometrically, blue colour (due to starch) changes to colourless when $I_{2}$ has completely reacted $2 S_{2} O_{3}^{2 -} + I_{2} \to S_{4} O_{6}^{2 -} + 2 I^{-}$ but after sometimes blue colour reappears.

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Solution

Iodine test, $I_{2} + s t a r c h \to$ blue colour.
But when iodine completely reacts, solution become colourless. After some time, the blue colour appears because some atmospheric oxidation has taken place, forming $I_{2}$ , which reacts with starch to give the blue colour again: $2 I^{-} + \frac{1}{2} O_{2} + 2 H^{+} \to I_{2} + H_{2} O$

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2 S_{2} O_{3}^{2 -} + I_{2} \to S_{4} O_{6}^{2 -} + 2 I^{-}

I_{2} + 2 S_{2} O_{3}^{2 -} \to 2 I^{-} + S_{4} O_{6}^{2 -}

2 S_{2} O_{3}^{2 -} + I_{2} \to S_{4} O_{6}^{2 -} + 2 I^{-}

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