is no color change with Benedict\'s or SeliwanofTs reagenk , but blue-black colo

Question

is no color change with Benedict's or SeliwanofTs reagenk , but blue-black color with iodine reagent -s E. Hydrolysis of Disaccharides and Polysaccharides Sucrose + H2O | Sucrose + HCI Starch + 11,0 Swemm + HC Resules i. Iodine test 2. Benedict's test 3. Hydrolysis (Yes/No) Questions and Problems Q6 Ho w do the results of the Benedict's test indicate that hydrolysis of sucrose and starch occurred? 07 How do the results of the iodine test indicate that hydrolysis of starch occurred? Indicate whether the following carbohydrates will give a positive (+) or a negative (-) result in each type of test listed below: 08 Benedict's Test Seliwanoffs Test Fermentation Test Iodine Test Glucose Fructose Galactose Sucrose Lactose Maltose Amylose Copyright © 2014 Pearson Education, Inc.

Explanation / Answer

6. Benedict's reagent is a complex mixture of sodium carbonate, sodium citrate and copper(II) sulfate pentahydrate. A positive test with Benedict's reagent is shown by a color change from clear blue to a brick-red precipitate. Sodium carbonate provides the alkaline conditions which are required for the redox reaction. Sodium citrate complexes with the copper (II) ions so that they do not deteriorate to copper(I) ions during storage.

Principle: When reducing sugars are heated in the presence of an alkali they are converted to powerful reducing species known as enediols. Enediols reduce the cupric compounds (Cu2+) present in the Benedict's reagent to cuprous compounds (Cu+) which are precipitated as insoluble red copper(I) oxide(Cu2O).

When Benedict’s solution and simple carbohydrates are heated, the solution changes to orange red/ brick red. This reaction is caused by the reducing property of simple carbohydrates. The copper (II) ions in the Benedict’s solution are reduced to Copper (I) ions, which causes the color change. The colour change in the case of sucrose and starch denotes that they must have get hydrolysed before reacting to the Benedicts reagent. As the complex sugars which do not have free aldehyde group cannot react with the reagent. So, if the colour change is observed, they must have hydrolyzed before reaction.

7. Reaction with Iodine: the colour change observed from brown to blue black indiactes that the hydrolysis of starch has been occured. The hydrolyzed starch will react with the triiodide ion.

8. glucose- +,-,-

Fructose- +,+,-

Galactose- +,-,-

Sucrose- -,+,-

Lactose- +,-,-

Maltose- +,-,-

Amylose- -,-,+

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