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a. Give a chemical test of starch with I...

a. Give a chemical test of starch with `I_(2)`.
b. What is the change in colour, when the test is performed at high temperature?
c. What structural changes occurs during the change in reaction conditions?
d. Do amylose and amylopectin give the same colour?

Text Solution

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a. Strach with `I_(2)` gives a deep blue-black colour.
b. On heating, the colour changes to reddish brown.
c. Amylose part in starch traps `I_(2)` molecule in its helix and forms a charge transfer complex which has blue-black colour.
At high temperature. the helix partially breaks and some `I_(2)` molecules escape and some `I_(2)` molecules get trapped. On cooling, the helix reforms. trpping more `I_(2)` molecules and the original colour returns.
d. Amylopectin gives a less intense red-brown colour because the helical structure is disrupted by the branching of the chain.
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