What happens to fish in the winter when the water freezes?
To answer this question, we first have to take a look at the chemical properties of water.
In general, most substances become more and more compact as they are cooled. However, the nature of water is such that, as it freezes, its molecules join into rings, each of which takes up more space than the same number of liquid molecules. This means that ice (frozen water) is less dense than liquid water. For this reason, ice is lighter than water and will, in fact, float on water (which is why icebergs float).
Now, think of what happens to a pond or lake as the temperature cools and the water begins to freeze. As ice crystals form, they float to the surface of the water. Eventually, the entire surface freezes over, covering the bulk of the water underneath, which is still liquid. Once the ice forms, it acts as insulation, helping to retain heat in the liquid below.
Thus, unless the temperature is extremely low for a very long period of time, the water below the ice does not freeze, although the ice may grow thicker. In a normal winter, the cold does not last long enough to freeze all the water, so the fish are able to live just fine until the coming of spring warms the surface and melts the covering of ice.
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