To Study Some Simple Tests of Carbohydrates

Introduction

Carbohydrates give your brain, kidneys, heart muscles, and nervous system energy. Fiber, for example, is a carbohydrate that aids digestion, makes you feel full, and aids in the control of blood cholesterol levels.

The word "carbohydrate" tells you exactly what it is. Carbohydrates have a carbon backbone to which hydrogen and oxygen molecules are attached. "Carbo" means carbon, and "hydrate" means water.

Monosaccharides are the simplest carbohydrates; they have the general chemical formula $${{(}{C}{H}_{2}{O}{)}_{x}}$$ and are known as simple sugars. The most common monosaccharides have three to six carbon atoms in a single-bonded unbranched chain. The suffix -ose denotes monosaccharides.

Table of Contents

• Aim of the Experiment

• Apparatus Required

• Theory

• Procedure

• Observations

• Result

• Precautions

Aim of the Experiment

To study some simple tests of carbohydrates.

Apparatus Required

• Glucose 

• Sucrose

• Lactose 

• Starch 

• Molisch’s Reagent

• Fehling’s Solution

• Benedict’s Solution 

• Iodine Solution

Theory

• Molisch test for carbohydrates – Molisch’s reagent + Given sample food → Purple or violet ring confirms the presence of carbohydrate.

• Fehling test for carbohydrates – Fehling’s reagent + Given sample food → Red precipitate confirms the presence of carbohydrates

• Benedict test for carbohydrates – Benedict’s reagent + Given sample food → Red precipitate confirms the presence of carbohydrates.

• Tollen test for carbohydrates – Tollen’s reagent + Given sample food → Silver mirror confirms the presence of carbohydrates.

• Iodine test for carbohydrates – Iodine solution + Given sample food → Blue colour solution confirms the presence of starch.

Procedure

Preparation of Reagents

• Molisch’s reagent – It is prepared by adding alpha-naphthol in 10% alcoholic solution.

• Fehling’s reagent – It is solution of 1 ml each of Fehling’s A and Fehling’s B

• Fehling’s solution A – In distilled water, dissolve copper sulphate and add a few drops of sulfuric acid.

• Fehling’s solution B – In 150ml of distilled water, dissolve sodium potassium tartrate and sodium hydroxide.

• Benedict’s reagent – 0.25g of anhydrous sodium carbonate in distilled water is added to a sodium citrate solution, and copper sulphate solution is added to it. Add distilled water to bring the total volume up to 125ml.

• Tollen’s reagent – To the silver nitrate solution, add solution of sodium hydroxide. Then, drop by drop, add ammonium hydroxide solution until the precipitate dissolves. Tollen’s reagent is the name given to the clear solution.

• Iodine solution – Dissolving iodine in potassium iodide solution yields iodine solution.

1. Molisch’s Test

• Add a few drops of Molisch's reagent (1% alcoholic solution of 1-naphthol) to 1-2 ml of aqueous carbohydrate solution (suspension in the case of starch). Put 1 mL of cone in. $${{H}_{2}{S}{O}_{4}}$$ is slowly poured down the side of the test tube.

• At the intersection of two layers, a red violet ring is formed.

2. Fehling’s Test

• Add 1—2 ml of Fehling's solution A and Fehling's solution B to 2 ml of aqueous carbohydrate solution (nearly 5%). Maintain the test tube in boiling water bath.

• The presence of a reducing sugar is indicated by the presence of a reddish precipitate.

3. Benedict’s Test

• To 2 ml of aqueous carbohydrate solution (nearly 5%), add 1—2 ml of Fehling's solution A and Fehling's solution B. Keep the test tube immersed in a boiling water bath.

• The presence of a reddish precipitate indicates the presence of a reducing sugar.

4. Tollen’s Test

• In a test tube, place 2-3 mL of carbohydrate aqueous solution. To it, add 2-3 mL of Tollen's reagent. For 10 minutes, place the test tube in a boiling water bath.

• The presence of reducing carbohydrates is indicated by a gleaming silver mirror.

5. Iodine Test (For starch only)

• Add 1-2 drops of iodine solution to the sample's aqueous suspension.

• The presence of starch is indicated by the presence of blue colouration.

Observations

Result

The given organic compound is a carbohydrate compound.

Precautions

• Handle acids with caution, such as concentrated sulphuric acid.

• When taking reagents from the reagent bottles, always use droppers.

• Heat the reaction mixture slowly and carefully.

Lab Manual Questions

1. How will you distinguish between sucrose and glucose?

Ans. Glucose on heating with Tollens' reagent gives silver mirror while sucrose does not.

2. Explain why does fructose reduce Fehling’s solution and Tollen’s reagent inspite of the presence of ketonic group?

Ans. In alkaline medium fructose rearranges to glucose and the two are equilibrium with each other.

3. What is the role of tartarate and citrate ions in Fehling’s reagent and Benedict’s reagent respectively?

Ans. Using Fehling's solution or Benedict's solution, Complexing the copper(II) ions with tartrate ions prevents precipitation of copper(II) hydroxide. Benedict's solution contains copper(II) ions complexed with citrate ions in sodium carbonate solution.

4. What are reagents used in this experiment?

Ans. Following are the reagents used in this experiment:

• Molisch's reagent

• Fehling's reagent

• Benedict's reagent

• Tollen's reagent

• Iodine Solution

Viva Questions

1. Which chemical is used for carbohydrates testing?

Ans. Concentrated sulphuric acid is used in testing carbohydrates.

2. Why is testing for carbohydrates important?

Ans. To characterize carbohydrates present in an unknown solution on the basis of various chemical assays.

3. What is the purpose of testing known samples for lipids carbohydrates and proteins?

Ans. To detect the presence of carbohydrates, fats and proteins that are present in the samples of the given food. 

4. Why are carbohydrates polar?

Ans. Carbohydrates are more polar than lipids due to (d) presence of many -OH groups in carbohydrates. 

5. What makes a carbohydrate a reducing sugar?

Ans. All those carbohydrates which contain a free aldehyde or ketonic group and reduce Fehling's solution and Tollen's reagent are referred as a reducing sugar. Examples of reducing sugar is galactose, glucose, glyceraldehyde, fructose, ribose, and xylose.

6. Where are carbohydrates stored?

Ans. The body breaks down most carbohydrates from the foods we eat and converts them to a type of sugar called glucose. Glucose is the main source of fuel for our cells. When the body doesn't need to use the glucose for energy, it stores it in the liver and muscles.

7. What are classification of carbohydrates?

Ans. Carbohydrates are divided into four types. They are 

• Monosaccharides 

• Disaccharides

• Oligosaccharides

• Polysaccharides

8. What are the benefits of carbohydrates?

Ans. Following are some benefits of carbohydrates

• Carbohydrates are your body's main source of energy.

• They help fuel your brain, kidneys, heart muscles, and central nervous system.

• For instance, fiber is a carbohydrate that aids in digestion, helps you feel full, and keeps blood cholesterol levels in check.

9. What are the main functions of carbohydrates?

Ans. The four primary functions of carbohydrates in the body are to provide energy, store energy, build macromolecules, and spare protein and fat for other uses.

10. How are carbohydrates formed?

Ans. Carbohydrates are produced in green plants through the process of photosynthesis. In this chemical reaction, plants are able to harness the sun's energy and store it within sugar molecules that are produced. Plants take in carbon dioxide, $${{C}{O}_{2}}$$ and water, $${{H}_{2}{O}}$$.

Practical Based Questions

1.Which of the following Biomolecules simply refers to as “Staff of life”?

(a) Lipids

(b) Proteins

(c) Vitamins

(d) Carbohydrates

Answer: (d)

2.Which of the following is the simplest form of carbohydrates?

(a) Carboxyl groups

(b) Aldehyde and Ketone groups

(c) Alcohol and Carboxyl groups

(d) Hydroxyl groups and Hydrogen groups

Answer: (b)

3.Protein, carbohydrates and fat are examples of Macronutrients

(a)True

(b)False

Answer: (a)

4.Which of the following monosaccharides is the majority found in the human body?

(a) D-type

(b) L-type

(c) LD-types

(d) None of the above

Answer: (a) 

5.Which of the following is the most abundant biomolecule on the earth?

(a) Lipids

(b) Proteins

(c) Carbohydrates

(d) Nucleic acids.

Answer: (c) 

6.Appearance of silver mirror in tollen’s test indicates presence of

(a)Acetaldehyde

(b)Acetone

(c)Formaldehyde

(d)Benzophenone

Answer: (a) and (c)

7.Which of the following is the smallest carbohydrate – triose?

(a) Ribose

(b) Glucose

(c) Glyceraldehyde

(d) Dihydroxyacetone

Answer: (c)

8.Which of the following is the simplest carbohydrate?

(a) Gulose

(b) Glucose

(c) Dihydroxyacetone

(d) Glyceraldehyde

Answer: (d)

9.Which of the following is a reducing sugar?

(a) Dihydroxyacetone

(b) Erythrulose

(c) Glucose

(d) All of the above

Answer: (d) 

10.Which of the following has reducing properties?

(a) Mucic acid

(b) Glucaric acid

(c) Gluconic acid

(d) Glucuronic acid

Answer: (d)

Key Features

Carbohydrates, also known as carbs, are sugar molecules. Carbohydrates are converted into glucose by our body. Glucose, also known as blood sugar, is the primary energy source for your body's cells, tissues, and organs. Fiber, for example, is a carbohydrate that aids digestion, makes you feel full, and aids in the control of blood cholesterol levels.

The word "carbohydrate" tells you exactly what it is. Carbohydrates have a carbon backbone to which hydrogen and oxygen molecules are attached. "Carbo" means carbon, and "hydrate" means water.