Question

The compound which gives a positive ninhydrin test and a negative Benedict’s solution test is:
A. A monosaccharide
B. A disaccharide
C. A lipid
D. A protein

Answer 1

Knowing the requirements of ninhydrin test and Benedict’s test is important to answer this question. Ninhydrin test checks the presence of amino acids and Benedict’s solution checks for the presence of reducing sugars.

Step by step answer: We will go step by step to understand and answer this question. We need to know the meaning of every word in the question and the answer to get to the answer. So, starting with the question, first let’s discuss the ninhydrin test.
Ninhydrin is a chemical which is used to detect the presence of ammonia or primary and secondary amines.
The development of deep blue or purple colour (which is also called Ruhemann’s purple) when ninhydrin is added to the solution of analyte indicates the presence of ammonia, amines or amino acids. It is most commonly used for the determination of fingerprints. It is a white solid and was discovered by Siegfried Ruhemann in 1910.

Benedict’s Solution is also called a Benedict reagent or Benedict qualitative solution. It is a complex mixture of Sodium carbonate, Sodium citrate and copper sulphate pentahydrate. It is used to test the presence of reducing sugars in a given analyte. Also, the presence of other reducing substances also gives a positive result. It can be detected by a colour change from clear blue to brick red with a precipitate.

A monosaccharide is also called simple sugar. They are the simplest form of sugar and are the most basic units of carbohydrates. They cannot be further broken down to simpler chemical compounds.

Disaccharide is a sugar formed when two monosaccharides are joined by glycosidic linkage. Both monosaccharides and disaccharides are soluble in water. Examples are sucrose, lactose and maltose.
Lipid is defined as a substance that is insoluble in water and soluble in alcohol, ether and chloroform. With carbohydrates and proteins, lipids are the main constituent of plant and animal cells.

Proteins are large size molecules formed by amino acids and 20 different types of amino acids exist in proteins. Hundreds and thousands of these amino acids are attached to each other in long chains to form a protein. It reduces sugar.
From the information on all the options given we get to know that proteins satisfy the tests of both ninhydrin and benedict’s solution as proteins are both amino acids and reducing sugar.

Therefore, option D is the correct answer.

Note: The colour change that happens during these tests indicated the presence of any compound. So, you need to remember the colour also. Like, in Ninhydrin test colour change is to blue or purple and in Benedict it is to brick red.