Most substances expand when heated and contract when cooled. Water also follows this rule, but only when the temperature is above 4 degrees Celsius. Actually water behaves the opposite at between the temperature from 0 to 4 degrees Celsius; it expands when the temperature ranges from 4 to 0 degrees Celsius.
This phenomena can be explained by the structure of water molecules. Water is tetrahedral in its electron pair geometry and its molecular geometry is bent. Water molecules are polar; its oxygen end is slightly negatively charged and the hydrogen side is positively charged. This makes the molecules to form hydrogen bonds among themselves. The hydrogen bonds become more stable when the temperature falls. And when water crystallizes, because of the intermolecular forces and the geometry of molecules it tends to form lattice that is full of holes. So the holes take up more spaces and make water expand when reaching 0 degree Celsius.
I find it interesting because I've known this phenomena since a child but I never knew the explanation for this unique behavior of water.
Submitted by Wae Nakayama
This is really cool! I've always wondered about this as well. I like how concepts we talked about in class help explain this process!
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