Differential Diffusion
Purpose: Diffusion is the movement of molecules, atoms, or ions from an area of higher concentration to areas of lesser concentration. The purpose of this experiment is to demonstrate that large molecules may be unable to diffuse across a semipermeable membrane whereas smaller molecules or ions can. In this investigation, the size of the membrane pore determines what can diffuse through it.
Materials and Methods: For this investigation the student will require a salt solution, a starch solution, a dialysis tube, rubber bands, a 500 ml or larger beaker of deionized water, several 50 ml beakers, 0.1M silver nitrate solution, and an iodine solution.
Wet the dialysis tubing so that it opens up. Tie off one end with a rubber band, and then fill the tube with equal amounts of salt and starch solution. Now tie the open end of the tube off with another rubber band. Wash the outer membrane with deionized water to remove any spilled solutions.
Experiment: Place the prepared bag in a beaker of
deionized water for about 30 minutes.
After 30 minutes test the deionized water using silver nitrate solution and iodine solution. Pour about 25 ml of the water into each of two 50 ml beakers and add a few drops of silver nitrate to one and a few drops of iodine solution to the other. Silver nitrate solution is a reagent that is used to test for salt in the water. If salt is present, the water turns cloudy when silver nitrate is added. Iodine is a reagent that is used to test for starch. If starch is in the water, a blue-black color will form.
Conclusion:
What was your observation after adding the reagents?
a. nothing happened in either
b. both a cloudiness and a blue-black color appeared
c. only a cloudiness was observed
d. only a blue-black color was observed
Based on your observation what is your conclusion?