Case: Observe the image given below. There are two test tubes (i) and (ii). Test tube (i) contains a starch solution, while test tube (ii) contains starch solution and saliva. Both the test tubes are kept undisturbed for about 20 min. Later, dilute iodine solution (brown in colour) was added to both the test tubes.

What results will we get after performing iodine tests on the contents present in both the test tubes?

Option Test tube (i) Test tube (ii)

A. Blue-black Brown

B. Brown Blue-black

C. Blue-black Blue-black

D. Brown Brown

[0.8 Mark]

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Solution

The correct option is A A
We know that blue-black colour is observed when dilute iodine solution is added to starch. Test tube (i) contains a starch solution, so when dilute iodine solution is added to it, a blue-black colour is seen. Test tube (ii) contains a starch solution and saliva. Saliva contains the enzyme salivary amylase. It breaks down starch into simple sugars. Iodine cannot form a complex with sugars, hence no blue-black colour is seen in the test tube (ii).

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Option	Test tube (i)	Test tube (ii)
A.	Blue-black	Brown
B.	Brown	Blue-black
C.	Blue-black	Blue-black
D.	Brown	Brown