Eli5: Materials expand when heated and contracts when cooled. So why does water expand when it freezes instead of contracting?
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Water does actually contract when it is cooled. However there is one exception: between about 4 C and 0 C, at atmospheric pressure, water will arrange itself into the lattice that is ice. This lattice is less dense so it expands until it freezes then it starts contracting again. And it’s important to note that the watcler is still “contracting” during that time, kind of, it’s just that the lattice it forms is less dense, so the molecules are getting closer together, but also rearranging themselves so there is more space at the same time.
Water is not the only substance that does this, but it is the only common one on earth that does it. And at different pressures the mechanics change.
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