Question

6) What would you observe in each of the following cases and provide an explanation. a) A dilute solution of iodine is added to an aqueous solution of starch. b) The solution is placed in boiling water. c) The solution is cooled in running water. d) An aqueous starch solution is treated with Fehling's solution and placed in boiling water. Would you expect the result to be the same as that for D- glucose? Explain. e) A stock solution of starch is mixed with 10% HCl and heated for 30 min. A sample is removed and mixed with iodine solution. f) The sample frome) is neutralized with NaOH to pH>7 and treated with Fehling's solution in boiling water.

Answer 1

a) Iodine forms a complex with starch molecule. And hence starch solution turns dark blue.

b) As the solution is placed in boiling water the starch is hydolyzed to oligo saccharides and monosaccharides. Hence iodine complex breaks and blue color dissapears.

c) Upon cooling the heated solution in running water will remain as it is. Because once hydrolyzed starch does not link back to form starch.

d) Upon heating in boiling water, starch from the solution is hydrolysed to give monosaccharide glucose molecules. These are reducable sugar molecules and hence Fehling's solution gives red ppt. This results are consistant with D-glucose. Because D-Glucose is also a reducing sugar.

e) Starch solution upon reaction with HCl it hydrolyses to glucose. Hence further addition of iodine solution does not give dark blue color. After hydrolysis the starch is absent and hence does not show complex formation with iodine.

f) After neutralization and reaction with Fehling's solution gives red ppt. Because the solution contains glucose and it is a reducing sugar.