The diffusion lab has been a yearly activity in my biology class as part of a unit on cells and cell transport. Students fill a bag with starch and water and then submerge it in a solution of iodine and observe what happens. The iodine diffuses across the plastic bag and turns the starch purple.

If students are absent for the lab, they can complete Google Slides that shows the step-by-step process of the investigation. They watch videos showing the set-up and observe a time-lapse video that shows the starch in the bag turn purple. Why does it turn purple?

Iodine in the beaker is a small molecule that can move through the plastic of the bag. When it encounters the starch solution, the color will change to purple. This is an excellent model for how diffusion occurs across semi-permeable membranes.

In the slide activity, a dialysis tube was used instead of a plastic bag because the process does occur faster with the dialysis tube. In class, I use a cheaper version, plastic sandwich bags. Save time and frustration by making the bags ahead of time. Simply put a spoonful of starch in the bag and fill with tap water. Tie the bag like a balloon. Students then place the bag into a beaker with a few drops of iodine.