Tests of Carbohydrates

Carbohydrates are the most abundant and diverse class of organic compounds occurring in nature. It played a key role in the establishment and evolution of life on earth by creating a direct link between the sun and chemical energy.

Aim:

To study some simple tests of carbohydrates in the given sample.

Theory:

The word carbohydrate is formed from the words carbon and hydrogen. Carbohydrates are combinations of the chemical elements carbon and hydrogen plus oxygen. In the natural world, carbohydrates are the most common chemical compounds used for food.

The following are the test to identify the presence of carbohydrates.

  1. Molisch’s test
  2. Fehling’s test
  3. Benedict’s test
  4. Tollen’s test
  5. Iodine test

(a) Molisch’s Test:

Molisch’s test is a general test for carbohydrates. This test is given by almost all of the carbohydrates. In this test concentrated sulfuric acid converts the given carbohydrate into furfural or its derivatives, which react with α-naphthol to form a purple coloured product.

The chemical reaction is given below.

Molisch's Test

Note: The appearance of purple or violet ring confirms the presence of carbohydrate.

(b) Fehling’s Test:

This test is given by reducing sugars. To the aqueous solution of carbohydrate fehling’s solution is added and heated in water bath. Formation of red precipitate confirms the presence of reducing sugars. The copper ions present in fehling’s solution in +3 state is reduced to +2 oxidation state and in alkaline medium it is precipitated as red cuprous oxide.

The chemical reaction is given below.

Fehling's Test

Note: The appearance of red precipitate confirms the presence of carbohydrates.

(c) Benedict’s Test:

This test is given by reducing sugars. in alkaline medium, sodium carbonate converts glucose to enediol and this enediol reduce cupric to cuprous forming cuprous hydroxide. This solution is kept in sodium citrate and on boiling red precipitate of cuprous oxide is formed.

The chemical reaction is given below.

Benedict's Test

Note: The appearance of red precipitate confirms the presence of carbohydrates.

(d) Tollen’s Test:

This test is given by reducing sugars. Carbohydrates reacts with Tollens reagent forms a silver mirror on the inner walls of the test tube. This confirms the presence of reducing sugars. Silver ions are reduced to metallic silver.

The chemical reaction is given below.

Tollen’s Test

Note: The appearance of silver mirror confirms the presence of reducing sugars.

(e) Iodine Test:

This test is only given by starch. Starch reacts with iodine solution forms complex blue colour solution. On heating the blue colour disappears and on cooling the blue colour reappears.

The chemical reaction is given below.
Iodine Test

Note: The appearance of blue colour solution confirms the presence of starch.

Materials Required:

  1. Molisch’s reagent
  2. Fehling’s reagent
  3. Benedict’s reagent
  4. Tollen’s reagent
  5. Iodine solution
  6. Concentrated sulfuric acid
  7. Sodium hydroxide solution
  8. Test tubes
  9. Test tube holder
  10. Test tube stand
  11. Bunsen burner
  12. Water bath
  13. Dropper
  14. Stirrer

Apparatus Setup:

Tests of Carbohydrates

Procedure:

Preparation of Reagents:

  • Molisch’s reagent – It is prepared by adding α-naphthol in 10% alcoholic solution.
  • Fehling’s reagent – It is a solution of 1ml each Fehling’s A and Fehling’s B
  • Fehling’s solution A – Dissolve copper sulfate in distilled water and add a few drops of sulfuric acid.
  • Fehling’s solution B – Dissolve sodium potassium tartrate and sodium hydroxide in 150ml of distilled water.
  • Benedict’s reagent – To a solution of sodium citrate 0.25g of anhydrous sodium carbonate in distilled water is added and copper sulfate solution is added to it. Make the total volume upto 125ml by adding distilled water.
  • Tollen’s reagent – Add sodium hydroxide solution to silver nitrate solution. Then add ammonium hydroxide solution dropwise till the precipitate dissolves. The clear solution is called Tollens reagent.
  • Iodine solution – Iodine solution is obtained by dissolving iodine in potassium iodide solution.

(a) Molisch’s Test:

  1. Take 2ml of given sample solution in a clean test tube.
  2. Add 2-3 drops of Molisch reagent slowly.
  3. Now add concentrated sulfuric acid along the sides of the test tube.
  4. The acid layer forms a layer at the bottom.
  5. Note the junction of the two layers.
  6. If there is formation of violet ring then the presence of carbohydrate is confirmed.

(b) Fehling’s Test:

  1. Take 2ml of given sample solution in a clean test tube.
  2. Add 2 ml of Fehling’s solution A and Fehling’s solution B to it.
  3. Keep the solution in a boiling water bath for about 10 minutes.
  4. If there is formation of red precipitate then the presence of carbohydrate is confirmed.

(c) Benedict’s Test:

  1. Take the given sample solution to be tested in a clean test tube.
  2. Add 5ml of Benedict’s reagent to it.
  3. Boil the solution for about 2 minutes.
  4. Cool the solution and observe the solution.
  5. If there is formation of green, red or yellow precipitate then there is presence of reducing sugars.

(d) Tollen’s Test:

  1. Take the given sample solution in a clean test tube.
  2. Add 2-3ml of tollens reagent to it.
  3. Keep the test tube in a boiling water bath for 10 minutes.
  4. If there is the appearance of shiny silver mirror confirms the presence of reducing sugars.

(e) Iodine Test:

  1. Take the sample solution to be tested in a clean test tube.
  2. Add 2-3 drops of iodine solution.
  3. Observe the change in colour.
  4. If there is the appearance of blue colour then the presence of starch is confirmed.

Observations and Inference:

Test Glucose Lactose Sucrose Starch
Molisch’s test Purple ring Purple ring Purple ring Purple ring
Fehling’s test Red precipitate Red precipitate No precipitate No precipitate
Benedict’s test Red precipitate Red precipitate No precipitate No precipitate
Tollen’s test Appearance of silver mirror. Appearance of silver mirror. No silver mirror No silver mirror
Iodine test No reaction No reaction No reaction Appearance of blue colour solution.

Results and Discussions:

The given organic compound is a _________ (reducing sugar/starch/carbohydrate) compound.

Precautions:

  1. Handle the acids like concentrated sulfuric acid with care.
  2. Always use droppers to take reagents from the reagent bottles.
  3. While heating the reaction mixture do it carefully.

Viva Questions:

1. What are carbohydrates?

Ans: Carbohydrates are polyhydroxy aldehydes, ketones or compounds that are transformed into one of a large group of organic compounds that occur in foods and living tissues, including sugars, starch and cellulose.

2. What is Feling’s solution?

Ans: Fehling’s solution is prepared by mixing two solution Fehling’s A and Fehling’s B. Fehling’s A contain copper sulfate solution whereas Fehling’s B contain potassium hydroxide and potassium sodium tartrate.

3. If a compound gives positive test for iodine test, what is the nature of the compound?

Ans: If a chemical compound give positive iodine test then the given organic compound is starch.

4. What are the most important constituents of food?

Ans: Carbohydrates, proteins and fats are the most important constituents of food.

5. What are reducing and non-reducing sugars?

Ans: Non-reducing sugars do not have an OH group attached to the anomeric carbon so that other compounds can not be reduced. Glucose is a reducing sugar. A disaccharide may be a reducing sugar or non-reducing sugar. Maltose is a reducing sugar, while sucrose is a non-reducing sugar.

Keep visiting BYJU’S to learn more about class 12 CBSE chemistry practicals.

Leave a Comment

Your email address will not be published. Required fields are marked *