Organic Chemistry

Hydrolysis of Starch

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Purpose: 

To see if starch breaks up into glucose molecules.

Materials:

starch solution, 10% iodine solution, 2M hydrochloric acid, 1M sodium hydroxide, Fehling’s solution A, Fehling’s solution B, 2 test tubes, test tube rack, hot water bath, pipettes, 24-well plate, small beaker with rinsing water, small waste water beaker

Safety:

Wear goggles and gloves.

Procedure:

  1. Start the hot water bath.
  2. Heat the test tube with the starch solution in the water bath for 1 minute.
  3. With a pipette transfer 10 drops of the heated starch/HCl solution to a well on the well plate.
  4. Rinse the pipette with distilled water.
  5. Test the solution in the well plate for the presence of starch.
  6. Repeat steps 2-5 until the solution no longer tests positive for starch.
  7. Add 10 drops of NaOH to the starch solution until the solution is slightly basic. (Place a drop of the solution onto Litmus paper. When the paper turns blue, the solution is basic.)
  8. Perform the Fehling’s test on the starch solution.
Observations:

After adding a significant amount of hydrochloric acid to the starch solution and heating the test tube for a while, the iodine proved that there was no longer any starch in the solution by staying a red/brown color in the solution.

            Once enough sodium hydroxide was added to the starch solution, it became slightly basic, turning the litmus paper blue.  The Fehling’s solution immediately became murky when the starch solution was added.  As it was being heated, the solution became slightly green.  Eventually, the murkiness cleared up a bit to a reddish blue solution with a bright red deposit on the bottom. 

                                                                           Discussion

This experiment showed that starch can be rehydrated with HCl and heat. This breaks the molecules up into single glucose molecules again (causing the positive result in the Fehling’s Test).  H+ molecules break up the glucose molecules. However, because these have strong bonds, heat is needed in order to create movement and weaken the bonds.  This same process can be applied to sucrose to achieve a similar result, but this will happen much faster, as there are not as many bonds in sucrose.  It will also work with cellulose, except the reaction will happen slower because of the many thick bonds of glucose.