Kids, did you ever hear of a crystal radio kit?  More on that in Part II of this activity, but here we will make an essential ingredient.  Rochelle salt can be used to grow very large single crystals that exhibit piezoelectricity.  This property means the creation of electricity resulting from pressure in the form of mechanical stress.  As a result these crystals can be used as transducers in microphones.  

Rochelle salt is also called potassium sodium tartrate with the molecular formula of KNaC₄H₄O₆•4H₂O.  Unless your adult partner works in a lab, you probably don't have this chemical lying around.  But you can make it in your own kitchen!

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

Make sodium carbonate (also called washing soda, Na₂CO₃) by having your adult partner heat the contents of one half of a box of sodium bicarbonate (also called baking soda, NaHCO₃) spread thinly in an oven-safe pan at 275°F for an hour.

Have your adult partner heat a mixture of about 80 grams cream of tartar (KC₄H₅O₆) in 100 milliliters of water to a boil in a saucepan.  Remove from heat.  One teaspoon at a time, slowly stir the sodium carbonate powder into the cream of tartar solution. The solution will bubble after each addition, so stir until bubbling stops. Continue adding sodium carbonate until no more bubbles form.

Chill this solution in the pan in the refrigerator. Crystalline Rochelle salt will form on the bottom of the pan. Carefully save your Rochelle salt aside for Part II of this activity which will appear in our next edition.

Notes: 

1.  Use a stainless steel, or better yet, a Pyrex saucepan -- not an aluminum pan to dissolve the cream of tartar.  The cream of tartar could leach some metal out of an aluminum pan which might interfer with your experiment.

2.  There are some examples of artificial cream of tartar on the market shelves;  check the lable to make sure the product you buy is the real thing.  

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
June 2012

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References: 
http://www.ehow.com/how-does_5467738_process-making-piezo-crystals.html

Anne Marie Helmenstine at About.com: Chemistry
http://chemistry.about.com/b/2011/05/05/how-to-make-rochelle-salt.htm?nl=1