Diffusion Lab – Iodine & Cornstarch

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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


  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:



5 thoughts on “Diffusion Lab – Iodine & Cornstarch

  1. Kayla January 6, 2022 / 1:50 pm

    Is there an alternative to iodine or do you know where to find cheap iodine? I don’t have access to a chemistry lab.


    • Liz Belasic January 6, 2022 / 3:11 pm

      I think I bought some from the supermarket or drugstore, sorry I do not recall.


  2. Christine Connolly October 7, 2018 / 6:58 pm

    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.

    Liked by 1 person

    • Liz LaRosa October 7, 2018 / 7:13 pm

      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 πŸ™‚

      Update – it was Wegmans brand, box was still in my room πŸ™‚


    • Anne October 20, 2021 / 2:08 pm

      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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