Qualitative Tests for Carbohydrates: Benedict Essay

Reducing sugar gives positive result in Benedict’s test as the colour of solution change from blue to brick-red and there is brick-red precipitate. Starch give positive result in Iodine test as the colour of solution change from yellow to dark blue. Glucose is the simplest carbohydrates that are monosaccharide or simple sugar. Starch consisting of a large number of glucose units joined together by glycosidic linkage and known as polysaccharide, that is polymers of sugar.

Saliva that contains salivary amylase enzyme, which breaks down the glycosidic linkage between glucose that found in starch and hydrolyzes starch into glucose at optimum temperature of 37°C, that is similar to human body temperature and show positive results to Benedict’s test The salivary amylase enzyme is denatured at 95°C as the enzyme lost in structure. In the experiment, test tube 1 and 4 is added with saliva and brown-orange solution with precipitate is present by test tube 1 at temperature of 37oC in the first 5th minutes as it is the optimum temperature for amylase enzyme to work.

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For test tube 4, orange solution is formed. After the incubation period, the amount of precipitate that present at the bottom of test tube 1 increases as more starch is hydrolyzed and green-blue solution is formed at test tube 4. Hydrochloric acid actives in high temperature but not room temperature. Hydrochloric acid hydrolyzes starch into glucose at high temperature of 95°C in order to show positive Benedict test.

At temperature of 37oC, hydrochloric acid may show a little or no reaction on starch as starch is not reacted at optimum temperature or the reaction is low. As the time of heating increases, the amount of starch that hydrolyzes by the enzyme increases. Hydrochloric acid is added into test tube 2 and 3. At the first 5th minutes, there not obvious change in test tube 2 and 3. After the incubation period, test tube 2 remains unchanged and test tube 3 changed colour to orange-blue solution that with precipitate.

Test tube 2 has no reaction as the temperature is low for hydrochloric acid to react efficiently. The starch in test tube 3 is hydrolyzed by hydrochloric acid to form glucose and positive result is obtained to Benedict’s test. At the beginning of the experiment, the samples of starch is advised not to put into the water bath of 95oC at the beginning as the glycosidic linkage between glucose may broken down by the high temperature. Sodium hydroxide is added to neutralize excess hydrochloric acid that remains in the solution as remaining of acid may ffected the final result of experiment. Thoughout the experiment, there are 2 temperatures of water bath as it used to show the effects on temperature to saliva, starch and also hydrochloric acid. At different temperature, there will different rate of reaction on different substance. Besides, all the volume of solution that used in the experiment must be similar to the volume propose to remain the accuracy of the result or the experiment.


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