When you heat up water it expands and turns to steam. When you freeze water it also expands. Why does ice expand?
IIRC, it expands because water has polar molecules. That is, one side has a slight positive charge, and the other has a slight negative charge. When water starts going below about 4 degrees Celsius, the molecules start to have a small enough momentum that the charge keeps them further apart from each other than when they were bouncing all over the place at higher temperatures.
You might want to check out this thread.
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