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:
- Place one bag over each beaker
- Add 1 tbsp of cornstarch to each bag
- Add 50 mL of water to each bag
- Check for leaks
- Use a rubber band on each one to keep closed
- Clip bag to beaker
Observations
- 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.
- Iodine is able to pass through the plastic bag, the starch is not
- Have students lift the bag out to see the changes that are taking place
- 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:
Middle School Science Blog 
Is there an alternative to iodine or do you know where to find cheap iodine? I don’t have access to a chemistry lab.
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I think I bought some from the supermarket or drugstore, sorry I do not recall.
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Hi, I’m just finishing up investigations with my 7th graders on osmosis and diffusion. Oddly, I tried this very set up hoping that inexpensive baggies would work. We had tried with a z-loc brand bags and had no luck ( just letters to the company letting them know how non permeable their baggies were) so I used the store brand baggie and we were still disappointed. I guess we all felt better about our sandwiches wrapped in these baggies but not to see the diffusion. We did a great investigation with carrots and salt and then moved onto “naked eggs”. I had been very hopeful the baggie would work. Glad it did for your crew.
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We used either Wegman’s brand fold over sandwich bags or ShopRite – I threw out the box without writing it down 😦 -Sorry it didn’t work for your class 😦
The carrots sounds like a great idea, I saw one using potato slices that I thought about using as well. I have never tried the egg one, but heard it works well 🙂
Update – it was Wegmans brand, box was still in my room 🙂
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Late to the party but I am using ziplock brand locking and it is working but I had to increase the amount of iodine to about 1/2 and 1/2 for it to work in a reasonable timeframe
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