A few drops of iodine fell on Paheli's socks and a few drops on her teacher's saree while experimenting with iodine in the lab. The saree got a blue-black spot where iodine fell while the colour of the socks did not change. Give the potential reason for this statement.

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Solution

Iodine test
Iodine-starch test is done to detect the presence of starch.
The principle involved in the iodine test is that amylose, a subunit of starch, is known to interact with iodine forming a blue-black colour complex.
The helical structure of amylose is known to form a charge-transfer complex with iodine, wherein iodine is found inside the helical structure of amylose.
Thus, the reason for the blue-black spot on the teacher's saree could be that the saree had been starched while Paheli's socks were starch free.
Hence, when the starched saree came in contact with iodine, a blue-black spot appeared.

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