Reactions of starch

2.  Experiment:
 Molisch’s test:
 1 ml starch + 2 or 3 drops of Molisch’s reagent.
 Mix well & add 1-2 ml Conc. Sulphuric acid along the
sides of the test tube without shaking.
 Observation:
 A reddish violet ring at the junction of two liquids.
 Inference:
 Starch is a carbohydrate.

4.  Composition of Molisch’s reagent:
 1% α- naphthol in 95% ethanol.
 Principle:
 Carbohydrates when treated with Conc.
Sulphuric acid undergo dehydration to give
furfural or furfural derivatives.
 These compounds condense with α- naphthol
to form reddish violet ring.

5.  General test for all carbohydrates
 Excess α- naphthol & impurities in reagent give green
colored ring.
 Molisch’s test is given by carbohydrates with at least 5
carbons.
 Trioses & tetroses do not answer this test.
 Interaction of acid & water produces heat & can cause
charring of carbohydrates (due to precipitation of
carbon) resulting in the formation of black ring.
 Acid should be layered very slowly.

6.  Experiment:
 2 ml starch + add 2-3 drops of iodine solution.
 Observation:
 Deep blue color
 Inference:
 Starch is a polysaccharide

7.  Principle:
 The amylose component of starch has helical
structure & when treated with iodine
solution, the iodine gets trapped inside the
helical structure giving the solution blue
color, which indicates the presence of starch.

8.  Experiment:
 5 ml Benedict’s reagent + add 8 drops of starch
solution.
 Boil it for 2 mints.
 Observation:
 No colored precipitate.
 Inference:
 Starch is a non-reducing sugar.

10.  Experiment:
 5 ml starch + add 4 drops of Conc. HCl + keep
in boiling water bath for 10 min + cool under
running water + neutralize by adding 5
drops of 10% sodium carbonate, till the
effervescence ceases.
 Inference:
 The solution is hydrolysed

11.  Experiment:
 5 ml Benedict’s reagent + 8 drops of
hydrolysate of starch, mix + boil for 2 min.
 Observation:
 Brick red precipitate.
 Inference:
 Hydrolysate contains reducing sugar.