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STARCH IN THE KITCHEN



PURPOSE

Determine whether starch is present in various kitchen products.
 

PERFORMING THE EXPERIMENT

1. Tear off a 30-centimeter (1 foot) piece of wax paper.  All chemical reactions will be performed using the wax paper as your container for the reactions.  Be very careful not to spill any of the samples off the wax paper. 

2. Remove very small amounts of various samples from their containers (either plastic bags or plastic vials).  For most of the samples, use a spoon to remove only enough that you can see easily.  For dog food, marshmallows, etc., use just 1 piece. (Note: Be sure to use both white rice and brown rice samples.)

3. Put each sample on the wax paper about 5 centimeters (2 inches) apart from each other.

4. Carefully place one drop of iodine solution from its dropper onto each of your samples.   Be very careful not to spill any iodine solution because it stains almost all materials including skin.  (It is not dangerous but often takes several weeks to  be washed off or to wear off.) 
The plastic medicine droppers are already filled.  Note that you let drops out of a dropper by gradually squeezing the plastic bulb.  NOTE:  Before you use an iodine dropper practice letting drops of water out of the PRACTICE MEDICINE DROPPER.

5. Note the color of each of your samples after adding the iodine. Which are simply stained brown by the iodine, and which take on a deep blue-violet or almost black color? 

6. Roll up your wax paper, and place it in a waste container.
 

QUESTIONS TO THINK ABOUT

1. Which classes of food products (breads, fruits, vegetables, or dairy products) do you think contain starch?  Which classes of food products do not contain starch? 
2.   Do white rice and brown rice react in the same way to the iodine solution?  Why or why not?
 

EXPLANATION

Starch is a large chemical molecule comprised of a long twisted chain with many branches.  When iodine is added, this long twisted chain captures the iodine inside its spiral pattern and forms an intense blue-violet (or sometimes almost black) color.  Thus, samples that contain starch turn a deep blue-violet color when iodine is added whereas samples that do not contain starch are simply stained by the brown iodine solution.
 

EQUIPMENT NEEDED

Plastic bags and/or glass vials containing various food products - e.g., sugar, flour, salt, cereal, coconut, dry dog food, small paper scraps, brown rice, white rice, miniature marshmallows, horseradish; iodine solution (1 part iodine from the pharmacy to 40 parts water) in droppers; wax paper; waste container.
 

REFERENCE

Modified from Janice Pratt VanCleave, Chemistry for Every Kid, 101 Easy Experiments that Really Work, Wiley, New York, NY, 1989, pages 104-107.
  

Kids Involved Doing Science is at http://www.kids.union.edu.
Modified 02/01/2001 by C. & P. Scaife