First-order transitions are the kind we're all familiar with: sharp and precise. Raise the temperature of an ice cube past 32°F, for example, and the change from ice to water happens all at once. Basically, what's going on is that the molecules are forced to make an either-or choice between order and chaos. At temperatures below the transition, they are vibrating slowly enough that they can make the decision for crystalline order (ice). At temperatures above the transition, however, the molecules are vibrating so hard that the molecular bonds are breaking faster than they can reform, so they
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