Seems counterintuitive?

[ballyhoo]

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Why do molecules expand when water freezes? I understand why the expansion of molecules is counteracted by an increase in pressure ... but :/

And what does it mean when it says, "Thus, freezing will only occur when the next lower temperature in equilibrium with such an increased pressure is reached. Thus, freezing point of water decreases when system pressure is increased."

Thanks!!

 

[ktran17]

Water is an exception because as temperature is lowered, the molecules slow down and thus are allowed to organize into a crystal lattice. However, unlike other solids, water's hydrogen bonding properties prevents it from consolidating closer, and therefore this is why ice is less dense than liquid water. D= mass/volume (volume increases and therefore D, density decreases)

There is a more complicated explanation to why ice is less dense than water but that's the general gist of it =)