Question from diffusion Lab experiment?

Question

In the diffusion Lab, the one where you put cornstarch in a bag, and Iodine surrounding it. Iodine has been in a continuous process of entering the bag. Yet according to diffusion, Iodine should stop entering the bag once the concentrations inside and outside the bag have become the same. How do you explain this? Details please, and easy to understand.

Answer 1

You are correct that if iodine concentrations inside and outside of the bag are the same then no net movement of iodine across this boundary will occur.  But, the cornstarch inside the bag acts as a large sink for iodine since starch binds this element.  Thus, iodine will be slowly taken up by diffusion of iodine from the gas phase inside the bag to the starch molecules.  So the iodine concentration inside the bag will always be a little lower than outside until the binding capacity of the starch for iodine has been exhausted.  Until this occurs the concentration difference (which may be quite small) will continue to drive net movement of iodine from outside to inside the bag.