Carbohydrates are polyhydroxy aldehydes or ketones and the substances which yield these on hydrolysis. Carbohydrates are classified as:

  1. Monosaccharides: glucose, fructose - (C6H12O6)
  2. Disaccharides: sucrose, maltose - (C12H22O11)
  3. Polysaccharides - starch, cellulose, glycogen

All the carbohydrates, except cellulose, are hydrolyzed in our body to glucose which on slow oxidation, in presence of oxygen, liberates large amount of energy. The oxidation of glucose (during respiration) is represented as:

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Thus, carbohydrates (sugars and starch) are the main source of energy in our body. The cellulose acts as a roughage. The carbohydrates which reduce Fehling’s solution and Tollen’s reagent are called reducing sugars. All the Monosaccharides and most of the disaccharides are reducing sugars. Sucrose (cane-sugar) is a non-reducing sugar and hence does not reduce Fehling’s solution.

Test for Carbohydrates

1. Molisch’s Test (for all carbohydrates)

When an aqueous solution or suspension of a sample containing a carbohydrate is treated with α-naphthol (Molisch’s reagent), followed by the addition of few drops of concentrated sulphuric acid along the sides of the test tube a red-violet ring appears at the junction of two layers. This happens because concentrated sulphuric acid converts carbohydrates to furfural or its derivatives which reacts with α-naphthol to give a coloured (red-violet) product.

2. Tollen’s Test (for reducing sugars)

Any aldose or ketose having a free carbonyl groups reduces Tollen’s reagent to metallic silver. This test is commonly known as silver mirror test as a kind of mirror is obtained on the walls of the test tube. The reactions involved in this test are:

3. Fehling’s Test (for reducing sugars)

When equal quantities of Fehling’s solution A and B are mixed with an aqueous solution of a reducing sugar and the mixture is heated on a boiling water bath, a reddish precipitate of cuprous oxide is formed. The following reactions are involved in the test.

4. Iodine Test for starch

Starch gives a purple-blue colour when treated with iodine. It is due to the adsorption of iodine on the surface of starch. The colour gets discharged on heating and reappears on cooling.

How To Perform Experiment

(i) Molisch’s Test

Take 2-3 mL of aqueous solution or suspension of the sample. Add a few drops of Molisch’s reagent and shake it. Then add 5-6 drops of conc. H2SO4 slowly along the side of the test tube.

(ii) Tollen’s Test

Take a test tube and clean it thoroughly with NaOH solution. Then wash the tube with excess of water to remove NaOH. Take 2-3 mL of aqueous solution of carbohydrate in the test tube and add 2-3 mL of freshly prepared Tollen’s reagent to it. Keep the test tube in a boiling water bath for about 10 minutes.

(iii) Fehling’s Test

Take a small amount of crushed food items (1 g) or 2-3 mL of aqueous solution of the sample (nearly 5%) . Add 2 mL each of Fehling’s solution A and Fehling’s solution B. Keep the test tube in a boiling water bath.

(iv) Iodine Test for Starch

Take a small amount (1-2 g) of the food sample in a test tube and add a few drops of dilute iodine solution.