|MadSci Network: General Biology|
This works because plastic has pores in it and thus the plastic bag acts as a semi-permeable membrane. The cornstarch particles are too large to pass through the pores in the plastic, they are large polymers of glucose. The iodine passes easily because it is a atom, and thus much smaller than the starch. Water molecules are too large too pass through the pores, and thus the only thing that can move across the membrane are the iodine ions. The iodine ions move about randomly, and in the course of this, some of them pass through the pores. Shannon Beck
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