Experiment 8. The hydrolysis of starch with hydrochloric acid
(a) Prepare a water bath by half filling a 250 cm3 beaker with warm water and heating it to boiling point on a tripod and gauze, with a Bunsen burner. When the water boils, reduce the flame to keep the water at boiling point.
(b) Label four test-tubes 1-4.
(c) Copy the table given below into your notebook.
(d) In each tube place 5 cm3 3% starch solution.
(e) Using a syringe or graduated pipette, add 3 cm3 Benedict's solution to the starch solution in
tube 1 and place the tube in the boiling water bath for five minutes.
(f) Rinse the syringe or pipette and use it to add 1 cm3 dilute hydrochloric acid to the starch solution in each of tubes 2, 3 and 4. Note the time and place all three tubes in the water bath.
(They will be removed at five, ten and fifteen minutes respectively).
(g) Remember to remove tube 1 from the water bath after five minutes if you have not
already done so.
(h) After five minutes, remove tube 2 from the water bath and cool it under the tap. Neutralize the acid by adding solid sodium bicarbonate, a little at a time, until the addition of one portion produces no fizzing. Place tube in the rack and return to tube 3.
(i) After ten minutes in the water bath, remove tube 3, cool and neutralize the contents as described in (h). Place the tube in the rack.
(j) After fifteen minutes in the water bath, remove tube 4; cool and neutralize as before, and place it in the rack.
(k) With a dropping pipette, remove a sample of the liquid from tube 2 and place 3 drops
on a spotting tile. Rinse the pipette and repeat the procedure for tubes 3 and 4. Add
one drop of dilute iodine to each drop of liquid on the tile.
(l) Rinse the syringe or pipette and use it to place 3 cm3 Benedict's solution in each of
tubes 2, 3 and 4. Return all three tubes to the water bath and heat for five minutes.
After this time replace the tubes in the rack and...