Question

Which of the following tests is performed by carbohydrates?
A. Biuret test
B. Nitroprusside test
C. Ninhydrin test
D. Molisch's test

Answer 1

Hint: Carbohydrates are biological molecules that are made up of carbon, oxygen and hydrogen; they have either aldehydes or ketones functional groups and are found in all living organisms.

Complete step-by-step answer: Among the given options, the Molisch test is given by carbohydrates, lets see what exactly the test is.
Molisch’s test is a general test for carbohydrates. Molisch’s test is a highly sensitive test for carbohydrates, and monosaccharide, oligosaccharides and polysaccharides all give positive Molisch's test. It is based on the reaction of the alpha-naphthol with the carbohydrate in the presence of sulphuric acid.
We should remember that this carbohydrate reacts with alpha-naphthol in an acidic medium to form a purple coloured solution. The intensity of colour increases with the increase in the amount of carbohydrate present in the solution. violet coloured ring is formed at the reaction of carbohydrate solution containing Molisch’s reagent and the sulphuric acid
The reaction is given below:

Now let’s see the other options also.
Biuret test is a chemical test which is used to detect the presence of peptide bonds and hence this test is specifically given by the proteins only. Therefore, this is an incorrect answer.
Ninhydrin test is used to check that whether a given analyte contains amine or alpha amino acids. At the end of the test the development of blue colour indicates the presence of amines, ammonia or alpha amino acids. Therefore, it is an incorrect option.
Nitroprusside test is used to detect the presence of keto acids in the blood and urine. The test measures only acetoacetate and acetone. Hence, this option is also incorrect.

Hence, the correct option is option D.

Note: Students should also note that there are many tests one can perform for the identification of carbohydrates other than Molisch’s test. Some of the test are -
a. Fehling's test
b. Iodine test
c. Benedict's test
d. JoEllen’s test