In this lab you will observe the diffusion of a substance across a semi permeable membrane.Iodine is a known indicator for starch. An indicator is a substance that chances color in the presence of the substance it indicates. Watch as your teacher demonstrates how iodine changes in the presence of starch.

Prelab Observations: Describe what happened when iodine came into contact with starch.


A.  Fill a plastic baggie with a teaspoon of corn starch and a half a cup of water tie bag. (This may already have been done for you)

B. Fill a beaker halfway with water and add ten drops of iodine.

C. Place the baggie in the cup so that the cornstarch mixture is submerged in the iodine water mixture.

D. Wait fifteen minutes and record your observations in the data table

E. While you are waiting, answer the questions.

We're going to think about concentrations now, which substances are more or less concentrated depends on which one has the most stuff in it.

1. Is the baggie or beaker more concentrated in starch?

2. Is the baggie or beaker more concentrated in iodine?

3. Define diffusion

4. Define osmosis

5. What is the main difference between osmosis and diffusion?


6. Why is iodine called an indicator?

7-8. Molecules tend to move from areas of _______ concentration to areas of  ______ concentration.

9. Make a prediction about what you think will happen after 15 minutes. 
    What will the colors be?


          Starting Color         Color after 15 minutes
Solution in Beaker 10. 11.
Solution in Bag 12. 13.


14. Based on your observations, which substance moved, the iodine or the starch? ________

15. The plastic baggie was permeable to which substance? ____________________

16. Why is it not a good idea to store iodine in a plastic bag?

Sketch the cup and baggie in the space below. Use arrows to illustrate how diffusion occurred in this lab.







From Diffusion Lab at the Biology Corner