Disappearing Crystals: A Refraction Experiment

Illustrate the idea of light refraction with this exciting and mystifying experiment. When placed in water, hydro-gel crystals will appear to disappear. Have your child conduct this experiment to find out why. (Hint: it's not magic, it's science!)

• 8 oz. clear plastic or glass jar or cup
• Distilled water
• Hydro-gel crystals (Purchased from a garden supply store or online at www.scienceinabag.com)
• String
   

Step 1
Fill an 8 oz. glass or cup 2/3 full of distilled water (regular tap water will also work). Place one crystal in the cup. Within 20 to 30 minutes the crystal will begin to swell. As the crystal grows it will begin to disappear under the water.

Step 2
Allow the crystal to sit undisturbed you can no longer see it. If bubbles form inside the crystal, place the cup in the refrigerator (not in the freezer) for a few hours to dissolve the excess gas.

Step 3
When the crystal has disappeared, carefully reach into the cup and fish it out. It will be visible once it is removed from water. Tie a string around the crystal, being careful not to tie the string too tightly otherwise the crystal will be cut in half!

Step 4
Lower the crystal back into the water using the string. The container will appear to contain only water and a small string noose hanging down into it. Lift the string out of the water. A large shiny crystal will be seen tied up in the noose! When lowered back into the water, the crystal again disappears!
 

Ask your child why it was possible to see the crystal in the air but not in the water. Encourage her to think critically about the experiment. When she's expressed her theories, whether they are accurate or not, review the concept of light refraction to illustrate exactly why the crystal appeared and disappeared. Here's what you need to know: