Theory & Procedure, Qualitative Analysis of Carbohydrates | Additional Study Material for NEET PDF Download

Our objective

Our objective is to study some simple tests of carbohydrates.

The Theory

What are Carbohydrates?

Carbohydrates are most abundant organic compounds found in living organisms and are composed of carbon, hydrogen and oxygen. Carbohydrates act as the primary source to provide energy for functioning of living organisms. These are called carbohydrates because they can be considered as hydrates of carbon. Most of them have the general formula Cx(H₂O)y.

Generally carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones or the compounds which produces such products on hydrolysis. Carbohydrates are called saccharides. Some of them have sweet taste and are called sugars.

How do you classify carbohydrates based on reactivity?

Based on the reactivity with Tollen’s, Benedict’s or Fehling’s reagent, carbohydrates are classified as;

Reducing sugars

Carbohydrates that can reduce Tollen’s, Benedict’s or Fehling’s reagents are called reducing sugars (sugar with free aldehyde or ketone group). All monosaccharides and most of the disaccharides are reducing sugars. Some examples are Maltose and Lactose.

Non-reducing sugars

Carbohydrates that cannot reduce Tollen’s, Benedict’s or Fehling’s reagents are called non-reducing sugars. Sucrose is a non-reducing sugar.

Some Important Tests for the Detection of Carbohydrates

Molisch’s test

Molisch’s reagent is 10% alcoholic solution of α-naphthol. This is a common chemical test to detect the presence of carbohydrates. Carbohydrates undergo dehydration by sulphuric acid to form furfural (furfuraldehyde) that reacts with α-naphthol to form a violet coloured product.

Fehling's test

This is an important test to detect the presence of reducing sugars. Fehling’s solution A is copper sulphate solution and Fehling’s solution B is potassium sodium tartrate. On heating, carbohydrate reduces deep blue solution of copper (II) ions to red precipitate of insoluble copper oxide.

Benedict's test

Benedict’s test distinguishes reducing sugar from non-reducing sugar. Benedict’s reagent contains blue copper (II) ions (Cu²⁺, cupric ions) that are reduced to copper (I) ions (Cu⁺, cuprous ions) by carbohydrates. These ions form precipitate as red coloured cuprous (copper (I) oxide. 

Tollen’s test

Tollen’s reagent is ammoniacal silver nitrate solution. On reacting with carbohydrate elemental silver is precipitating out of the solution, occasionally onto the inner surface of the reaction vessel. This produces silver mirror on the inner wall of the reaction vessel.

Iodine test

Iodine test is used to detect the presence of starch. Iodine is not much soluble in water so iodine solution is prepared by dissolving iodine in water in presence of potassium iodide. Iodine dissolved in an aqueous solution of potassium iodide reacts with starch to form a starch/iodine complex which gives characteristics blue black colour to the reaction mixture.

Learning Outcomes

• Students understand various identification tests for carbohydrates.
• Students acquire the skill to distinguish reducing and non-reducing sugars.
• Students acquire skill to perform the experiment in the real lab.

Materials Required

Samples

• Glucose
• Sucrose
• Starch

Reagents

• Distilled water
• Molisch’s reagent
• Fehling’s solution A
• Fehling’s solution B
• Benedict’s reagent
• Tollen’s reagent
• Iodine solution

Apparatus

• Test tube
• Dropper
• Beaker

Procedure

Real Lab Procedure

Solubility test

Take a small amount of the given sample in a test tube and add 3 ml of distilled water into it and shake the test tube well.

Observation

Inference

Glucose and sucrose are soluble in water whereas starch is insoluble in water.

Molisch’s test

Take about 2 ml of aqueous solution of the sample in a test tube and add few drops of Molisch’s reagent into it. Pour 1 ml conc. H2SO4 slowly along the side of the test tube.

Observation

Inference

Presence of carbohydrate is confirmed.

Fehling’s test

Take 2 ml of aqueous solution of the sample in a test tube and add 1-2 ml each of Fehling’s solution A and Fehling’s solution B into it. Keep the test tube in a boiling water bath.

Observation

Inference

Reddish precipitate indicates the presence of a reducing sugar.

Benedict’s test

Take 1-2 ml of aqueous solution of the sample in a test tube and add 1-2 ml of Benedict’s reagent. Keep the test tube in a boiling water bath.

Observation

 Inference

Reddish precipitate indicates the presence of a reducing sugar.

Tollen’s test

Take 2-3 ml of aqueous solution of the sample in a test tube and add 2-3 ml of Tollen’s reagent. Keep the test tube in a boiling water bath for 10 minutes.

Observation

Inference

A shinning silver mirror indicates the presence of a reducing sugar.

Iodine test

Take a small amount of aqueous solution of the sample in a test tube and add 1-2 drops of iodine solution into it.

Observation

Inference

Appearance of blue colouration indicates the presence of starch.

Simulator Procedure (as performed through the Online Labs)

You can select the test from ‘Select the test’ drop down list.

Solubility Test

• Drag the dropper towards the test tubes to drop water into test tubes A, B, C and D respectively.
• Drag the test tubes A, B, C and D respectively to shake the test tubes well.
• Click on the inference icon to see the inference.

Molisch’s Test

• Drag the dropper towards the test tubes to drop Molisch’s reagent into test tubes A, B, C and D respectively.
• Drag test tube A towards the reagent bottles to place it in a slanting position.
• Drag the dropper towards test tube A to drop Conc. H₂SO₄ slowly along the side of the test tube.
• Drag the test tube B towards the reagent bottles to place it in a slanting position.
• Drag the dropper towards test tube B to drop Conc. H₂SO₄ slowly along the side of the test tube.
• Drag the test tube C towards the reagent bottles to place it in a slanting position.
• Drag the dropper towards test tube C to drop Conc. H₂SO₄ slowly along the side of the test tube.
• Drag test tube D towards the reagent bottles to place it in a slanting position.
• Drag the dropper towards test tube D to drop Conc. H₂SO₄ slowly along the side of the test tube.
• Click on the inference icon to see the inference.

Fehling’s Test

• Drag the dropper towards the test tubes to drop Fehling’s solution A into test tubes A, B, C and D respectively.
• Drag the dropper towards the test tubes to drop Fehling’s solution B into test tubes A, B, C and D respectively.
• To place the test tubes in a water bath, click on the button ‘Place the test tubes in water bath’.
• Click on the inference icon to see the inference.

Benedict’s Test

• Drag the dropper towards the test tubes to drop Benedict’s reagent into the test tubes A, B, C and D respectively.
• To place the test tubes in water bath, click on the button ‘Place the test tubes in water bath’
• Click on the inference icon to see the inference.

Tollen’s Test

• Drag the dropper towards the test tubes to drop Tollen’s reagent into test tubes A, B, C and D respectively.
• Click on the ‘Next’ button to go to the next step.
• To place the test tubes in water bath, click on the button ‘Place the test tubes in water bath’
• Click on the inference icon to see the inference.

Iodine Test

• Drag the dropper towards the test tubes to drop iodine solution into test tubes A, B, C and D respectively.
• Click on the inference icon to see the inference.

To redo the experiment, click on the ‘Reset’ button.

Note: Click on the ‘HELP’ button to see the instructions.

Precautions

• Handle the chemicals with care.
• Heating should be done very carefully.
• Use droppers to take reagents from the bottles.