h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Why does water expand when it freezes, while most other thing shrink? (besides the obvious answer of 'so it can blow the cork off of the bottle of wine you put in the freezer for a chill; and forgot about.')

I don't know myself but I'm sure my husband will - he's a refrigeration engineer - but he's gone to get his hair cut so I'll get back to you on that

Pip

I'm back and so is himself. He says it's because water crystalises and becomes a solid. When it crystalises it forms pockets with gases trapped within them and therefore it appears to take up more room but if the crystals could be compacted together without pockets they would take up less room.

Apparantly this is because Hydrogen and Oxygen have different boiling and freezing points and therefore fight against each other in the freezing process which forces the elements to crystalise.

He also says if you add ice cubes to a glass of water, when they melt the water would be at the same level.

I'm sorry, but that's wrong. It has to do with the fact that water is a polar molecule. This means that one end has a slight negative charge and the other end has a slight positive charge. Therefore, when it is solidified (frozen) it only has one structure that it can take on, as same charges repel. This structure is a hexagonal crystalline matrix, which has empty space in it, although it does not contain gas. The space between the molecules causes the water to expand as it freezes. The reason water isn't always that density is that as a liquid the molecules are free to roam about and bounce around, but the freezing process jams them together and forces the crystalline structure.

At school our physics teacher (phil findley, if your out there hi, you were an inspiration) told us that if two liquids, one hot and one colder, placed in a freezer (ice cream making was the example given), then frequnetly the warmer one would freeze quicker. Apparently this was "discovered" by an african school boy, whose teachers disbeilived him, but was proved right by a visiting egg head.

Is this theory correct (Mr Findlay had a wierd sense of humour and we never knew when he was being less than truthful).?

CH

My old Physics teacher said the same thing, but he was weird too so he could have been kidding. The explanation was that the hotter liquid supercools below the freezing point (ie the temperature drops rapidly and 'overshoots' the freezing point). Then, instead of the 'cold' liquid slowly freezing over, something in the 'hot' liquid triggers off crystallisation and the whole lot freezes solid at once. I've seen this happen in one of those double skinned beer glasses (that contain a liquid which you freeze to keep your beer cold) - I opened the freezer, saw it hadn't frozen, picked up the glass and the liquid went solid instantaneously.

CS.

The polar argument is basically correct. like everything else water gets denser as it cools. Until 4 degrees celsius at which point it expands again, forming solid ice. There are actually lots of different structures for ice. Ridiculous amounts of money is spent on finding (or proving or something) Ice VII (i.e. the seventh different form of ice) The form that ice takes is dependant on the temperature, and pressure, at which the ice is formed. Its also relevant to that supercooling argument (also incidentally correct) as different structures form then too.
My facts may not be entirely right though as I had a bottle of (uncooled) wine over lunch.

Have a look at http://math.ucr.edu/home/baez/physics/hot_water.html

I heard one explanation for this, which is that some of the hot water
evaporates, leaving less water to actually cool (and also cooling the
rest of the water in the process, compare to sweating). I don't
understand the supercooling argument (as described here at least)
since logically any hot water needs to be cooled down to become "cool"
water before it freezes. Except that temperature isn't really
continuous but only a statistical value.. maybe someone could describe
it in more detail?

many things in life are counter-intuitive what we think is not the case. try the brain teaser regarding doors on researcher 26199's home page here;

http://www.h2g2.com/U26199

I can assure you the answer given is correct and can prove it but it does not sit easy on the mind, some people I have asked it refuse to beleive the answer.

Chris