Observations and results
Did smaller, more abundant crystals form in the jar and on the string in the bowl of ice water, whereas larger, fewer, better-shaped crystals formed in the jar at room temperature?
As the hot, saturated mixture of Borax and water cooled, there was more borax than could be contained by the colder water, and so this borax fell out of the mixture and formed crystals. A crystal is made of molecules of a product that have come together in a specific repeated pattern. When the molecules of the crystal come together, other products that are often considered impurities, or the unwanted products of the chemical reaction, do not fit well into the structure, much like the wrong piece of a puzzle does not fit. If the crystals form slowly enough, the impurities will be rejected because they do not fit correctly, and instead will remain in the water. This is why the crystals in the room-temperature jar should have been larger and more cube-like. But if a solution is cooled too quickly, there isn't time to expel the impurities and instead they become trapped within the crystal structure and the pattern is disturbed. Consequently, the crystals in the bowl of ice water should have formed more quickly and in greater numbers, but were smaller and less cubelike in shape because they had more impurities.
More to explore
Crystallization , from the Department of Chemistry and Biochemistry at the University of Colorado at Boulder
How to Grow Great Crystals: Tips, Tricks and Techniques , from About.com
Crystal Chemistry ( pdf ), from the Royal Society of Chemistry
Crazy Crystal Creations: How to Grow the Best and the Largest Crystals , from Science Buddies
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