Johnson, Betty Haugan Elementary School
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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?
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