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.

Table of Contents

• Aim
• Theory
• Materials Required
• Apparatus Setup
• Procedure
• Observations and Inference
• Results and Discussions
• Precautions
• Frequently Asked Questions on Tests of Carbohydrates

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 compound used for food.

The following are the tests 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.

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. The 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.

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

(c) Benedict’s Test:

This test is given by reducing sugars. in an alkaline medium, sodium carbonate converts glucose to enediol and this enediol reduces 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.

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

(d) Tollen’s Test:

This test is given by reducing sugars. Carbohydrates react with Tollens reagent and 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.

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.
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:

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 of 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 up to 125ml by adding distilled water.
• Tollen’s reagent – Add sodium hydroxide solution to the 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 the 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 a formation of the 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 the 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 a blue colour then the presence of starch is confirmed.

Observations and Inference:

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.

Also, Check ⇒ Structure & Properties of Maltose

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