Eli5: Materials expand when heated and contracts when cooled. So why does water expand when it freezes instead of contracting?
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It’s not a perfect rule that materials expand when heated and contract when cooled, just an observation.
What’s going on is when water freezers it’s molecules arrange themselves into a crystal structure, meaning each molecule enters into a *fixed spot* in relation to the molecules around it. Liquid water has no such structure, each molecule floats freely compared to those around them.
It just happens that with water the arrangement of fixed spots are kind of far from each other, at least farther apart then the liquid water molecules are on average. Thus, as water freezes and the molecules move into their crystal slots, it expands a bit.
The underlying reason for this boils down to the structure of the atoms that make up water molecules, the arrangement of the electrons within the atoms, and the resulting “shape” of the crystals these molecules form. It’s actually pretty complicated but also regular and predictable, if you ever decide to take college level science courses, Material Science is a whole area of study the covers crystalline solids and how they structure themselves.
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