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During candy making, a single sugar crystal can cause an entire batch of sugar syrup to crystallize, just as a single ice crystal in this activity causes an entire water drop to freeze.
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| What Do I Need? | . |
| • a bowl, at least 4 inches in diameter | |
| • clear plastic wrap | |
| • an eye dropper | |
| • purified water (bottled water is cleaner than tap water) | |
| • a freezer | |
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• a timer
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| What Do I Do? |
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1. Stretch the plastic wrap over the mouth of the bowl. |
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| 2. Use the eye dropper to place six or more small drops of water, 1/8 inch in diameter, on the clear wrap. | |
| 3. Place the bowl with the water drops in the freezer, and set the timer for 5 minutes. | |
| 4. Look in the freezer. Notice that some of the drops are opaque because they are crystalline solids, while others are transparent liquid. | |
| 5. Touch the transparent liquid drops with a piece of ice from the freezer, or sprinkle some ice crystals scraped from the wall of the freezer onto the drops. | |
| 6. Notice that they rapidly freeze solid. | |
| What’s Going On? | . |
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It is possible to cool purified water to temperatures below freezing (32° F) and have the water remain liquid—this is called supercooled water.
The addition of a seed crystal to supercooled water will trigger the formation of ice crystals.
The trick here is that the water must be clean. Any impurities in the water drops, such as dust particles, can trigger ice crystal formation, preventing the water from becoming supercooled. The smaller the drop, the greater the chance it will not have an impurity. The drops that solidified all contained impurities that triggered ice crystal formation. The drops that stayed liquid were all free of impurities.
Experiment with the amount of time you keep the water drops in the freezer before examining them. The exact amount of time depends on the temperature of your freezer and the size of your water drops. You want to find a time so that some of the drops freeze solid and a few remain liquid. |
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