Crystallization and Instant Ice

By Tracy V. Wilson, HowStuffWorks.com

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Rock candy might be the ultimate indulgence for a sweet tooth. But if you ever have a hankering for this pure-sugar confection and decide to make your own, you're in for a wait. It takes about a week, and it only works if you've started with perfect proportions of sugar and water. It's almost as bad as if you have a thirst for a tall glass of ice water and discover your ice cube trays are empty.

Rock candy and ice cubes both come from the same process -- crystallization. A crystal is basically a substance that has molecules that line up in a regular pattern. As a result, visible collections of those molecules -- like chunks of rock candy -- have the same patterned appearance.

Crystal formation tends to take a while. First, at the microscopic level, molecules have to start clumping together. This is known as nucleation, and often it takes place in the presence of an impurity, the way snowflakes form around dust in the atmosphere. But crystal nuclei aren't very stable, and they'll fall apart or dissolve back into the surrounding liquid if they don't reach a critical mass. But once the nucleus is big enough, the crystal can grow until it's visible with the naked eye.

There are a couple of ways to speed up this process. One is by supercooling a completely pure sample of a liquid, like water. Since it's completely pure, there's nothing for those first few molecules to latch onto to form a nucleus. Shake the water up, though, and tiny bubbles form inside -- just what the atoms need to start nucleation. When this happens, the water turns to ice almost instantly.

Another way is by making a supersaturated solution of water and sodium acetate. You may have seen this before in hand warmers that heat up and solidify but are reusable if you boil them. In this case, sodium acetate is dissolved in warm water until the water can't hold any more -- it's saturated. As the water cools, the sodium acetate becomes a little more resistant to being dissolved, and the water becomes supersaturated. Add just one crystal to this solution, and the sodium acetate will organize itself and crystallize in the blink of an eye. You can't eat it like rock candy, but it's exothermic, so it'll keep your feet warm.