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Disappearing Crystals: A Refraction Experiment

What You Need:

• 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

What You Do:

1. Fill an 8-ounce 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.
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.
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!
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!

What Happened?

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:

Refraction is the bending of light waves caused by a change in their speed as they pass through different materials, such as air and water. The amount of bending that occurs depends on the speed of light in both materials. The greater the difference between the speeds of light in two media, the more the light is bent as it passes at an angle from one medium to another. You are familiar with this phenomenon: it’s what causes a spoon in a glass of water to appear bent, and makes sunset last a few seconds longer as light from the already set Sun refracts around the Earth through the atmosphere.

The hydro-gel crystal is super absorbent, and capable of holding up to 99 percent water. When submerged in water, is made up almost entirely of water. The reason the crystal disappears when placed in water is because the crystal refracts the light the same as water and therefore cannot be seen while in the water. When the crystal is lifted out of the water and into the air, it becomes instantly visible because the air’s refraction of light is very different then that of the water!

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