What is the purpose of testing water in the reaction with Seliwanoff\'s reagent

Question

What is the purpose of testing water in the reaction with Seliwanoff's reagent and with the iodine solution? A solution in the lab was known to be either glucose or fructose. Which test could be used to correctly identify the solution? Iodine reacts with starch to form a blue-colored complex. Explain the results of the iodine test with the hydrolyzed starch solution. Sucrose differs from honey in that honey is a mixture of glucose and fructose. Sucrose is a disaccharide made up of them. Explain why honey is a quicker source of energy than sucrose.

Explanation / Answer

Ans 3. These reagents are used to check that if the distilled water contains starch or sugars in it. This to test for the starch it is treated with the iodine solution. If starch is present in the water then a blue colored complex is formed in the solution and if the water is free from starch then the solution remains colourless. Similarly to test the presence of a sugar Seliwanoff's reagent is added to the water and a red color is formed when a ketose sugar is present or light pink color is formed when an aldose sugar is present. If no sugar is present then the solution again remains colorless.

Ans 4. Glucose is an aldose and fructose is a ketose thus, to differentiate them in laboratory Seliwanoff's reagent is used. It is the mixture of resorcinol and concentrated hydrochloric acid. This reagent when heated with the glucose which is an aldose gives a red color solution rapidly but when this reagent is heated with fructose which is an aldose the solution turns to light pink in color slowly.

Ans 5. When starch solution is unhydrolysed then it forms a blue-black colored complex with iodine solution. But when the starch solution is hydrolysed that is cleavage of glycosidic bonds to give smaller carbohydrates then on addition of iodine solution does not form a blue-black colored complex, only red or golden color of iodine solution is seen in the solution.

Ans 6. Honey being a mixture of monosaccharides that is, glucose and fructose is readily absorbed in the bloodstream and provide instant energy from glucose and fructose. In case of sucrose which is a disaccharide has to be digested first to break the glycosidic bond to give monosaccharides and then these monosaccharides provide the energy. Thus, honey provides an instant source of energy as compared to sucrose.

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