This month's Challenge Question:
Frozen Pond
We know that at a temperature of
approximately 4° C, water begins to expand
before freezing (0° C). Why is this fact important for the fish in a
pond?
And the answer is:
Most liquids have a quite simple behavior when they are cooled (at a fixed
pressure): they shrink. The liquid contracts as it is cooled; because the
molecules are moving slower they are less able to overcome the attractive
intermolecular forces drawing them closer to each other. Then the freezing
temperature is reached, and the substance solidifies, which causes it to
contract some more because crystalline solids are usually tightly packed.
Water is one of the few exceptions to this behavior. When liquid water is
cooled, it contracts like one would expect until a temperature of approximately
4 degrees Celsius is reached. After that, it expands slightly until it reaches
the freezing point, and then when it freezes it expands by approximately 9%.
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