Chemist the Mr.
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If I freeze water, it expands, potentially breaking its container. If I evaporate water, it expands, potentially breaking its container (for the sake of discussion, the container in question is made of some boro-silicate or whatever). Why would water expand when it freezes? Doesn't that mean the particles are moving less? I can understand why particles that move more would need more space, but particles that move less? I DEMAND ANSWERS!
When water freezes it forms a rigid lattice structure which happens to be less dense than the original water, unlike in most cases when the solid is more dense. See the Wikipedia article on Ice#Characteristics.
What he said.
Though if you want to add an amusing factoid on a related subject), frozen water expands and therefore floats. The water displaced by an iceberg is exactly equal to the liquid volume of the ice in question (because displacement is based on mass). Therefore, when sea ice forms and melts on a seasonal basis, sea levels aren't affected.
Thanks. Is it possible to freeze water without the lattice? Is that what happens if you freeze it under high pressure (which would also raise the freezing temperature, right)?
Well, seawater is denser than fresh water (enough to affect the loading capacity of cargo ships - see wp:Waterline#Standard load line marks for an amusing bit of trivia), so an iceberg displaces a slightly smaller volume of seawater than it would if it was floating in fresh water.