MOLECULAR SHUFFLE

Johnson, Betty                Haugan Elementary School
9653 So. Merrion              (312) 478-0680
Chicago, Illinois   60617
(312) 994-6912

Objective:

  To observe how a selectively permeable membrane works.

Materials:

  cellophane dialysis tubing
  glass rod
  string
  starch suspension
  80% glucose solution
  distilled water
  3 beakers
  medicine droppers
  2 test tubes
  Benedict's solution
  Bunsen burner
  tripod
  iodine
     
Strategy:

      Soak a 20-cm section of cellophane dialysis tubing in water for 
a few minutes.  Rub the ends between your thumb and index finger until 
the ends separate.  Insert a glass rod to hold it open.  Twist one end 
and tie it tightly with string.  Remove the glass rod and then fill 
the dialysis bag 3/4 full with starch suspension and 1/4-full with 80% 
glucose solution.  Tie the top of the bag with string.  Leave a loose 
piece of string 10-15 cm long.  Rinse the outside of the bag with 
distilled water.  Leave the string outside the beaker so that you may 
remove the bag from the water.  After 20 minutes withdraw a dropper 
full of water from the beaker.  Place this liquid in a test tube and 
add 10 drops of Benedict's solution.  Heat in a boiling water bath for 
10-15 minutes. 
  
     While the water bath is boiling, place 30 drops of the liquid 
from the beaker in a test tube. Add three drops of iodine.  Observe 
the color of the water after the iodine has been added.  (If it turns 
blue or black, then starch is indicated.)  Now add 20 drops of iodine 
to the water in the same beaker and observe any color changes.  Report 
your results by answering questions 1-8. 

Evaluation:

  The students should answer the  following questions:
 
  1.  What happens when you heat Benedict's solution and some water 
      from the beaker?
  2.  Explain your results?
  3.  Do you note any changes in the amount of liquid in the beaker?
  4.  What conclusion can you draw from this observation?
  5.  What is the purpose of adding iodine to the liquid from the 
      beaker?
  6.  What happens when you add iodine to the liquid from the beaker?
  7.  Describe any changes observed when iodine was added to the beaker?  
  8.  If a change has occurred, what does this indicate?