Weekend Science Fun: Iodine Chemistry

As mentioned earlier, this year is the International Year of Chemistry. To celebrate, we are going to have a series of fun chemistry activities and experiments for children.

1. Iodine as a Starch Indicator

What is an indicator? In chemistry it is a substance that “indicates” the presence or state of another substance, often by changing color. For example, pH indicators change color in the presence of substances with certain levels or ranges of pH.

The element iodine can be used to indicate the presence of starch, a complex molecule often found in plants or foods derived from plants. Let’s take a look at how that works (with the supervision of an adult).


  • tincture of iodine (available at pharmacies)
  • disposable containers such as paper plates or bowls
  • newspaper (or other disposable material) to cover work surface
  • safety:  gloves, shoes, old clothes
  • food to test:  apples, onions, potatoes, corn chips, bread, corn starch, etc.
  • knife or other implement to cut food

Note about safety:  read all the product warnings on the label before using. Iodine is used as a disinfectant, but it can stain skin and clothes (it can also be toxic in higher concentrations.) Be sure to wear gloves and closed-toe shoes when working with it. Clean up spills and dispose of all food used in this demonstration immediately and completely so the tested items will not be accidentally consumed by humans or pets.

Cut up a small sample of each food to be tested and place in a disposable container on a work surface covered with newspaper or other disposable material. Predict whether each food item contains starch. You might want to write down your predictions. Now place a few droplets of iodine on each item. Iodine is normally a yellow-brown color. If the iodine turns a dark blue-black color, starch is present.

Your results may look something like this:

If you don’t try it yourself, this video shows what you might see: (wish he was wearing gloves)

2. Are plastic bags impermeable? An iodine test.

Now we are ready to find out how well plastic bags keep substances in or out. This experiment is a modified version of a demonstration used to show how materials diffuse in and out of cells through a membrane.


  • tincture of iodine (available at pharmacies) – see safety precautions above
  • corn starch
  • water
  • Mixing container and implement for corn starch and water
  • 4 beakers or large disposable containers for liquids that will hold at least 1 full cup of liquid
  • 3 different types of sandwich and/or zipper- lock style bags (make one a freezer bag, if possible) and twist-ties

First, cover your work surface with newspaper. Mix about 1/3 cup of corn starch in about one pint of water in a container. Next fill each of the beakers with 1/2 cup water. Add drops of iodine to each of the 4 beakers of water until the color is a golden brown. Try to add the same amount to each one.

Now, add 1/2 cup of the cornstarch-water mixture to each of the three bags. Also add 1/2 cup to the 4th beaker to serve as a control. Seal the bags with the zipper-lock or with a twist -tie. Suspend each bag in a beaker, with the cornstarch mixture into the iodine water. Be careful not to overflow the beaker. Now wait and record what happens every five minutes.

Your experiment should look something like this, although you should avoid our mistake and start with bigger containers. 🙂

After 1/2 hour you should see some differences. Can you tell the regular sandwich bag from the heavy-duty freezer bag? What happened to the corn starch?

Be sure to wear gloves when you dispose of the containers and clean up your work surface.

Do you think things would be different if you put the iodine in the bag and the cornstarch-water in the beaker? See this video for the answer.

Let me know if you have questions or comments.

For more high school/college level videos and information see the NKU Demo Database- Chemistry.

Upcoming International Year of Chemistry Events are appearing on the website. For example:

Super Science Saturday: Celebrating Chemistry – Past, Present, Future, a science extravaganza for the West Michigan community.  This event will be held on Saturday, January 29, 2011, at Grand Valley State University.

Southwest Georgia
The chemistry celebration will kick off at 9 am February 12, 2011 at Bailey Science Building of Valdosta State University with a morning of hands-on science activities for students of all ages and their parents.

The American Chemistry Society (ACS) is putting together a IYC 2011 -Calendar although the events link is not active as of today.


  1. Magic and Mayhem

    I was looking for the instructions for the starch test today and stumbled upon your blog. I love the baggie experiment even more than the first. Great stuff. Thanks! Off to wander around some more. 🙂

  2. Roberta

    Oh my, I wandered around your blog, too. I love the cecropia project! (Which I will add here for those who a curious: http://magicandmayhem.homeschooljournal.net/2013/10/26/our-newest-visitor/ )

  3. Roberta

    And oh yes, hope you have fun with the starch!

  4. Liam

    I guess I could try some of this ARGO* Corn starch

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