Science Mystery: Testing the Mystery Powder to Identify It

First read this about starch testing  (and have your students test some things in your kitchen for starch if you would like.) Once they understand starch testing in general, we can look at this testing we did for this mystery.

For the starch test, like many science experiments, we needed to start out with some knowns. We knew that the white powder was either cornstarch or baking soda, so we got out some cornstarch and put it in a cup and marked it so we wouldn't get it mixed up.

Then we got out some baking soda and did the same with it.

I gave my sleuths the mystery powder in another cup.

Iodine can be used to test for the presence of starch. Let us first see what it looks like when it is dropped on the cornstarch and the baking soda and see if there is a difference.

It might be hard to tell from these photos but the iodine stayed the same brown color when dropped on the baking soda, but turned a bluish-black color when it touched the cornstarch. The presence of starch makes the iodine change color.

So what happened when it hit the mystery powder?

It turned black! Now we know that it is a starch. It must be cornstarch, not baking soda! Who does this clue point us to?