Tests of Carbohydrates

For the students of Class 12 Science Chemistry, it is important to do the practical project of the subject. Chemistry is the subject for which possessing only theoretical knowledge is not sufficient at all, and hence the practical understanding of the subject is a must for all the students. And is, for this reason, the practical project work is included in the syllabus of Class 12 Chemistry. And hence, to make it easy Vedantu provides the students with the Tests of Carbohydrates practical for the same.

An Overview of the Practical Carbohydrates Test that Vedantu Provides

In the syllabus of Class 12 Chemistry, the Chapter on “Biomolecules” is included. In which the topic of Carbohydrates is covered, which includes the Classification of Carbohydrates, that is to say, Aldoses and Ketoses, then there come the monosaccharides and D-L Configuration. All of these are important for the students to understand before starting the practice of Carbohydrates. And hence first of all Vedantu provides a bird’s eye view of all these topics for a quick revision, before diving into the practical project.

There are various types of Tests for finding the presence of Carbohydrates, and the project that Vedantu provides for the Class 12 Chemistry practical Carbohydrates, covers three of them. Which are Molisch’ Test for Carbohydrates, Benedict’s Test for Carbohydrates, Iodine Test for Carbohydrates.

The project that Vedantu provides includes the Principle and requirements for all the types mentioned, along with a brief explanation of the same. Vedantu provides the project for the Test of Carbohydrates, for the Class 12 Chemistry practical in such a manner that it becomes easy for the students to understand and comprehend.

Also, if you wish to revise your understanding of the topic, you may find this link helpful: Class 12 Chemistry Revision Notes for Chapter 14 - Biomolecules (vedantu.com)

What are Carbohydrates?

Carbohydrates are one of the major Classes of biomolecules besides proteins and lipids. From a tiny grain of rice to the thick woody barks of trees, carbohydrate is omnipresent in one form or the other, forming the structural backbone of most life forms on earth. A molecule that is so fundamental to our existence has been and is still studied extensively. In the process, the need for Tests of Carbohydrates arose and thus, came about the development of chemical Tests for Carbohydrates. 

The identification Test for Carbohydrates exploits the chemical structure of sugars to detect their presence or absence in the Test solution. Our aim here is to study some carbohydrate Test methods like Molisch's Test, Benedict's Test for Carbohydrates, Iodine Test for Carbohydrates, and their underlying Principles and the carbohydrate Test procedures.

The Tests for the presence of Carbohydrates involve chemical Tests of Carbohydrates. The ones we will discuss here are:

1. Molisch's Test for Carbohydrates

2. Benedict's Test for Carbohydrates

3. Iodine Test for Carbohydrates

Molisch's Test

• Principle: It is a general Test for Carbohydrates. In Molisch's Test experiment, Carbohydrates are reacted with Molisch's Reagent and concentrated sulphuric acid; the former dehydrates to form furfural and its derivatives. The products further react with sulfonated alpha-naphthol to give a purple coloured complex. All Carbohydrates, that is, monosaccharides, disaccharides, and polysaccharides give a positive result for Molisch's Test.

• Requirements: Sample, distilled water, Molisch's Reagent, concentrated sulphuric acid, sulfonated alpha-naphthol, Test-tubes, Test-tube stand, Test-tube holder, and dropper.

What is Molisch's Reagent?

The Molisch's Reagent consists of α-naphthol dissolved in ethanol.

• Molisch's Test Procedure

1. Take 2 mL sample in clean Test-tube and 2 mL distilled water in another as control

2. Add 2-3 drops of Molisch's Reagent to the tubes

3. Gradually, add concentrated sulphuric acid along the inner walls of the Test-tubes

4. The formation of a violet ring at the junction of the liquid layers confirms the presence of carbohydrate

Benedict's Test

• Principle: The Principle is that when a solution of reducing sugar is heated with Benedict's Reagent, the alkaline sodium carbonate converts the sugar into enediol* and this enediol further reduces the cupric ions of the Reagent into cuprous ions. The resulting precipitate of cuprous oxide is brick-red in a colour that confirms the presence of reducing sugar. Lactose, maltose, and glucose give a positive reaction to this Test.

• Requirements: Sample, distilled water, Benedict's Reagent, Test tubes, Test-tube stand, Test-tube holder, Bunsen burner, and dropper.

What is Benedict's Reagent?

Alkaline Benedict's solution is used to Test the presence of reducing sugars. It has the following components:

1. Anhydrous Sodium Carbonate – imparts alkalinity to the reaction medium

2. Sodium Citrate – forms a complex with cupric ions so that they are not reduced to cuprous ions during storage

3. Copper (II) Sulphate Pentahydrate – gives the cupric ions

4. Distilled water- used as a solvent

• Preparation of Benedict's Solution: To prepare a litre of Benedict's Reagent, mix 173 g sodium citrate, 100 g anhydrous sodium carbonate, and 17.3 g copper (II) sulphate pentahydrate in 1000 mL of distilled water.

• Benedict's Test Procedure

1. Add 2 mL of Benedict's Reagent to 0.5-1 mL of the sugar solution (and distilled water as control)

2. The Test tubes are heated for 3 to 5 minutes

3. The appearance of brick-red precipitate confirms the presence of reducing sugar

Iodine Test

Principle: The Test gives positive results only with polysaccharide starch. The Principle is that on reacting with starch, Iodine gets trapped in the helical coils of the polysaccharide chain via a coordinate complex**. Due to the complex formation, blue/black colour is observed, which confirms the presence of starch. The blue/black colour disappears on the addition of an alkali or heating; this is due to the uncoiling of the polysaccharide network and release of the Iodine molecules.

Requirements: Sample, distilled water, Iodine solution, Test-tubes, Test-tube stand, Test-tube holder, Bunsen burner, and dropper.

Iodine Test Procedure

• Take 2 mL sample in clean Test-tube and 2 mL distilled water in another as control

• Add five drops of Iodine solution to the Test-tubes

• A blue/black colour confirms the presence of starch and a yellow/brown colour its absence

Advantages of Having the Explanation of the Test of Carbohydrates for Class 12 Chemistry Practical

The students need to have a practical understanding of Chemistry because it helps the students in understanding the subject in a better manner. Also, if students have done the practical work in a perfect manner, then it also becomes easy for the students to remember the whole process, and hence the practical work is very important for the students.

But it is always a good thing for the students to have some sort of guidance in regards to the project work. Because there are many things which the students have to take care of while performing the project. And hence Vedantu provides to the Class 12 Chemistry students the Test of Carbohydrates for the Chemistry practices. It includes all the necessary things that the students are supposed to know for the practical work. And since it is prepared by expert educators it becomes easy for the students to understand it.