Please anyone tell me. CLASS 7


Ans 1.1. Chewed bread does not contain starch so when we add iodine to it, it shows no change in colour. Whereas, when we add iodine to unchewed starch then it turns blue-black. 

Chewed bread does not contain starch , rather contain digested form of  starch which is a simple sugar. Starch is digested into simple sugar in mouth through the enzyme called salivary amylase which is found in saliva. In absence of starch, iodine does not change its colour. Chemical  formula of starch is (C12H22O11)n, while that of glucose is
(C6H12O6). Starch is a polysaccharide of glucose units,  joined by glycosidic bonds. Starch contains two molecules helical amylase and branched amylopectin. The iodine molecule slips inside of the amylose coil giving the blue black colour. While simple glucose which is digested form of starch does not contain amylase so does not give blue black colour to iodine.

Ans 1.2. Similar changes would be observed in chappati and crushed grams.

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