Question

Starch changes colour and turns blue black in ……….. solution?
A. Salt
B. Eosin
C. Iodine
D. Dilute HCl

Answer 1

Hint: To differentiate starch from glucose or to check its presence a test is done with one of the chemicals which is found in our food and is also important for our body. When starch reacts with that chemical it gives a blue black colour.

Complete answer:
Starch is a polysaccharide produced by most of the plants for the purpose of storage of energy. It is made up of several glucose units that are bound by glycosidic bonds. Starch consists of two types of molecules: the branched amylopectin and helical amylose. Amylopectin comprises more mass of starch near about 75-80% and the rest belongs to amylose.
To check for the presence of starch iodine solution is used as it instantly shows colour in the presence of iodine. Amylose present in starch is the reason for change in its colour to blue black. Iodine is not soluble in water so it is dissolved along with potassium iodide that forms a triiodide molecule which enters the helical structure of amylose and changes the colour of starch.
Potassium iodide and Iodine solution is yellowish orange and when it is dropped on any substance having starch then it converts to blue-black solution indicating the presence of starch.

Hence, the correct answer is option (C).

Additional information:
-The blue black colour that has developed due to the reaction of starch with iodine disappears when the starch solution is heated.
-In Iodometric titration starch is used as an indicator as starch absorbs iodine by capturing it in the helical amylose structure.
-The starch and iodine solution for the starch and iodine test should be freshly prepared or else their sensitivity for each other will decrease.

Note: This test for starch with the help of iodine shows a change in colour of the solution and shows the present of starch. This is a method of qualitative analysis of starch but is unable to tell us about the quantitative analysis of starch in a solution, that is the amount of starch present.