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FARGO — The freezing of open water in a lake or stream is complicated. In general, liquid water contracts as it cools and so becomes more dense, causing colder water to sink. However, at 39.2 degrees Fahrenheit, water reaches maximum density. Below 39.2 degrees, water begins to form a lattice structure and starts to expand and become less dense. Water freezes at 32 degrees and continues to grow less dense as its temperature drops.
This explains why a frozen container of water can burst. This property also keeps ice afloat in water. If ice grew more dense as it got colder, lakes would be far more likely to freeze solid, which would make it very difficult for fish and other animals to survive our cold winters. Water is not the only substance with this sort of relationship between density and temperature, but it is the one most important to life on Earth as we know it.
