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Johnson, Betty Haugan Elementary School
To observe how a selectively permeable membrane works.
cellophane dialysis tubing
80% glucose solution
2 test tubes
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
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.
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
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?