Kids, did you know that chemical reactions are going on all the time right in your mouth, especially when you chew? Let's see what happens to starch in our mouths.

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

Cut two one-inch squares from a piece of white bread. Put one square in your mouth and chew it about 30 times, making an effort to mix as much saliva as possible with the bread. When the bread is nice and mushy, spit it out onto a piece of wax paper. Put the other dry piece of bread onto a separate piece of wax paper. Add four drops of tincture of iodine to each bread sample. What happens? The unchewed bread turns a dark blue-purple color. The chewed up bread-saliva mixture does not turn dark.

Why? The starch in the bread combines with iodine to form iodine-starch molecules, which are blue-purple in color. Chewing the bread mixes it with saliva. This is where the chemical reaction occurred in your mouth: the large starch molecules were changed into completely different, smaller, sugar molecules. Sugar does not react with iodine in the same way that starch does. Chemists can use iodine as an "indicator" for starch. Try testing for starch in other foods, like a cracker, a piece of dry cereal, or a cookie!

(Safety Tip: Do not eat anything that has the iodine in it).

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
November 1994

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Reference: Janice VanCleave's "Chemistry for Every Kid" 1989, p.108.