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Materials and Set Up – this was so easy and inexpensive to do and had the same effect as using dialysis tubing. Great demo/lab as part of our unit on osmosis and diffusion!

For every two students:

• handout from Biology Corner
• large beaker
• inexpensive sandwich bag – non sealing (I used Wegmans 150 ct)
• 1 tbsp corn starch
• 50 mL water
• rubber band
• clothes pin
• graduated cylinder
• 100 mL Iodine dilution

Iodine Preparation

• 20 ml Iodine added to 500 mL of water
• measure out 100 mL of diluted iodine for each group

Prelab Prep:

1. Place one bag over each beaker
2. Add 1 tbsp of cornstarch to each bag
3. Add 50 mL of water to each bag
4. Check for leaks
5. Use a rubber band on each one to keep closed
6. Clip bag to beaker

Observations

1. Students will add iodine and make observations – changes will take place within a few minutes and the longer it sits, the darker it will become.
   1. Iodine is able to pass through the plastic bag, the starch is not
2. Have students lift the bag out to see the changes that are taking place
3. Discuss

Update – I let the set up sit over the weekend, and when I came in today, the water was almost completely clear – looks like just about all of the iodine moved into the bag: