Ghost Crystals (Demonstration)
(Reference: Robert Becker, Kirkwood High School, Kirkwood, MO)
 

Light :  Observation : Physical Properties : Polymers
 

Purpose:
Demonstrates to the students the idea of super absorbancy.  These polyacrylamide particles will absorb 500 times their own weight.  The dehydrated crystals can be used by florists in planters, they are also used in fire fighting and in diapers.
 

Safety:
Safety goggles and chemical resistant gloves should be worn by demonstrator
 

Materials:
Polyacrylamide crystals, can be purchased or extracted from a super abosorbant diaper
Petri dish
Large glass jar with lid
Plastic or glass vial with cap
Distilled water
 

Procedure:
Place a few crystals in a jar of distilled water and allow them to absorb the water for a couple of days.  Remove from the jar and tie a string around each one.  Then suspend the string and crystal into a jar of water, affixing the string to the lid.  Fill the jar with distilled water, then with the string and crystals submerged it will appear as though the string loop is empty!  Can let students observe, then open lid and slowly remove crystal and explain.

 
Explanation:
Since the crystal has absorbed 500 times its weight, it is now mostly water  so it has nearly the exact refractive index of water.  Therefore, when it is suspended in water you cannot see the crystal in the string loop.
 

Clean-up:
You can strain the crystals and used them in your potted plants at home, or you can save the crystal in the jar indefinitely.  Would recommend storing the jar and crystal in the fridge though.
 

Ginger Chateauneuf, 2000.
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