Forensic Chemistry – identify the mystery substance.

I saw this really cool Whodunit? experiment on Fetch! With Ruff Ruffman on PBS kids a while back. I’ve been dying to do it ever since and I finally got around to doing it with the kids. We made a substitution to the directions on the link: instead of grape juice, we used a homemade red cabbage indicator dye.

The object is to identify the mysterious unknown white powder by comparing the chemical reactions to three other known white powders (flour, baking soda and baking powder). By determining the reactions of the known, and comparing them to the unknown white powder, we should be able to determine the mystery substance.

Materials Needed

  • Data table (3 rows by 4 columns)
  • Pencil
  • Red cabbage juice
  • White vinegar
  • Iodine
  • 3 dishes, each with 1 teaspoon of baking powder
  • 3 dishes, each with 1 teaspoon of white flour
  • 3 dishes, each with 1 teaspoon of baking soda
  • 3 dishes, each with 1 teaspoon of the mystery substance (psst: put baking powder in it but don’t tell the kids)
  • 3 pipettes or eyedroppers

To make the red cabbage juice:

1. Slice 3-4 leaves of red cabbage into thin strips

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2. Add hot water to the cabbage in a glass jar (pickle jars work nicely).

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Let set for about 10 minutes to extract the purple color. You can mix it periodically

3. Strain the cabbage juice using a strainer and then you have your red cabbage indicator dye (why yes, it is supposed to be purple).  Oh, and you only need a little bit for this experiment, but you can save the rest of the cabbage juice for identifying household acids and bases.

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To identify the mystery substance:

1. Lay out your 12 dishes in a grid fashion like so

first column = flour
second column = baking powder
third column = baking soda
fourth column = mystery substance:

2. And place your chemical reagents in next to each row:

First row = cabbage juice indicator
Second row = vinegar (yes, apple cider vinegar works just as well as white vinegar)
Third row = iodine tincture (available at the pharmacy)

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3. Add 5-10 drops of your reagents to each dish one at a time, starting with the first column.

4. Record your results on your data table for each substance. “In some cases, a chemical reaction will occur. Signs of a chemical reaction include foaming, fizzing, or a change in color. But sometimes no chemical reaction can be seen. Can you tell the difference?”

5. Repeat this for each column of substances, and compare the reactions of the unknown white powder to the known substances

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When you are done, your child should be able to tell you what the unknown substance is.

For a printable/downloadable version of this experiment and a data table, click on download or print on the image below

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