Re: Why does iodine pass through a plastic bag?

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