Diffusion Lab

In this lab, students can see firsthand the diffusion of a substance across a semipermeable membrane.


Set-Up(can be done by teacher or student)
Fill beaker about halfway and put about ten drops of iodine. Put about a teaspoon of starch and about 50 ml of water in baggie and tie. (Actual measurements can vary) Put the baggie in the beaker and wait. In about 15 minutes the solution in the baggie will turn purple as a result of the iodine diffusing through the bag. You may want to discuss with students beforehand that iodine is an indicator for starch.

Possible discussion questions:

  1. The plastic bag is permeable to which substance?
  2. Why did the iodine enter the bag?
  3. Why didn't the starch enter the beaker?
  4. How is the plastic bag like the cell membrane?

Teacher Notes:During the fifteen minute wait period, you may want to discuss the definitions of diffusion and osmosis. You could also discuss what students think will happen?

Comments? Questions??
Curt Muskopf