Question

Two tubes labelled ‘P’ and ‘Q’ contain food stuff. Tube ‘P’ gave positive test with Benedict’s solution while tube ‘Q’ gave positive test with Nitric acid. Which of the following tests is correct?
a. Tube ‘P’ contains sugar, tube ‘Q’ contains protein
b. Tube ‘P’ contains protein, tube ‘Q’ contains sugar
c. Both tube ‘P’ and tube “Q’ contain sugar
d. Both tube ‘P’ and tube “Q’ contain protein

Answer 1

Benedict's reagent changes the color of the solution to orange-red/ brick red precipitate. The red-colored cuprous oxide is insoluble in water and hence, separate from the solution. It can be used to test for the presence of glucose in urine. Xanthoproteic acid gives yellow colors which are formed due to nitration of certain amino acids, most common examples are tyrosine and tryptophan.

Complete answer:
Some sugars like glucose are known as reducing sugars because they have the potential of transferring hydrogens (electrons) to other compounds by a process called reduction. When reducing sugars heated with the basic solution of Benedict's reagent containing copper sulfate enediols are formed.

Endioles react with cupric ions in Benedict’s solution and reduce cupric ions to cuprous ions. As Benedict's solution is heated with the reducing sugar the color changes from green to yellow to orange. Finally, red color is obtained. Hence tube 'P' contains sugar.

The presence of protein soluble in a solution is detected by using concentrated nitric acid. The nitric acid test is used for the detection of proteins. The amino acids in proteins such as tryptophan and tyrosine react with nitric acid to give the yellow coloration due to the formation of xanthoproteic acid. Hence tube 'Q' contains proteins.

Hence, the correct answer is option (A).

Note: When Benedict’s reagent is mixed with reducing sugars and heated, a reduction reaction causes Benedict’s reagent to change color. The color has a variation from green to dark red (brick) or rusty-brown, depending on the amount of and type of sugars. When you drop nitric acid on your nails, it turns yellow after some time, indicating the presence of keratin protein.