Tests of Carbohydrates: Procedures, Observations, Precautions and FAQs

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• Purpose of the Study
• Conceptual Framework
• Required Materials
• Setting Up the Apparatus
• Experimental Procedure
• Observations and Conclusions
• Results and Discussion
• Safety Precautions

Purpose of the Study:

The study aims to conduct simple tests to identify the presence of carbohydrates in a provided sample.

Conceptual Framework:

The term carbohydrate is derived from the words carbon and hydrogen. Carbohydrates are compounds made up of the chemical elements carbon, hydrogen, and oxygen. They are the most common chemical compound used for food in the natural world.

Further Reading: Viva Questions on Tests of Carbohydrates

The following tests can be carried out to detect the presence of carbohydrates.

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

(a) Test Using Molisch’s Reagent:

Molisch’s test is a general test for carbohydrates. Almost all carbohydrates give a positive result for this test. 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 outlined below.

Important: The formation of a purple or violet ring confirms the presence of a carbohydrate.

(b) Fehling’s Test:

This test is performed on reducing sugars. The carbohydrate solution is added to Fehling’s solution and heated in a water bath. The formation of a red precipitate indicates the presence of reducing sugars. The copper ions present in Fehling’s solution in the +3 state are reduced to the +2 oxidation state and in an alkaline medium, they precipitate as red cuprous oxide .

The chemical reaction is outlined below.

Important: The formation of a red precipitate confirms the presence of carbohydrates.

(c) Benedict’s Test:

This test is performed on 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 upon boiling, a red precipitate of cuprous oxide forms.

The chemical reaction is outlined below.

Important: The formation of a red precipitate confirms the presence of carbohydrates.

(d) Tollen’s Test:

This test is performed on reducing sugars. Carbohydrates react with Tollens reagent and form 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 outlined below.

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

(e) Iodine Test:

This test is specific to starch. Starch reacts with iodine solution to form a complex blue color solution. Upon heating, the blue color disappears and reappears upon cooling.

The chemical reaction is outlined below.
Important: The appearance of a blue color solution confirms the presence of starch.

Required Materials:

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

Setting Up the Apparatus:

Experimental Procedure:

Preparation of Reagents:

• Molisch’s reagent – This is prepared by adding α-naphthol to a 10% alcoholic solution.
• Fehling’s reagent – This is a solution of 1ml each of Fehling’s A and Fehling’s B
• Fehling’s solution A – Copper sulfate is dissolved in distilled water and a few drops of sulfuric acid are added.
• Fehling’s solution B – Sodium potassium tartrate and sodium hydroxide are dissolved in 150ml of distilled water.
• Benedict’s reagent – Sodium carbonate is added to a solution of sodium citrate in distilled water, then a copper sulfate solution is added. The total volume is made up to 125ml by adding distilled water.
• Tollen’s reagent – Sodium hydroxide solution is added to the silver nitrate solution. Then, ammonium hydroxide solution is added dropwise until the precipitate dissolves. The clear solution is called Tollen's reagent.
• Iodine solution – This is obtained by dissolving iodine in a potassium iodide solution.

(a) Test Using Molisch’s Reagent:

1. Take 2ml of the given sample solution in a clean test tube.
2. Slowly add 2-3 drops of Molisch reagent.
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 Tollen’s reagent to it.
3. Keep the test tube in a boiling water bath for 10 minutes.
4. If there is the appearance of a shiny silver mirror, it 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 Conclusions:

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 Discussion:

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

Safety Precautions:

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

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