Probably been asked before... Why does water expand....

[Mr_Cheesy]

... when it freezes, when most things contract ? I seem to remember it being something to do with the shape of the molecule, but it was a long time ago so it's entirely possible I've made that up... Thanks

[Clanad]

Water does contract as it coold, but only down to about 36 degrees F or so, at which temeprature it begins to expand and, upon freesing expands by nearly 10%. Our friends at Science Today explain "...There is a strong tendency to form a network of hydrogen bonds, where each hydrogen atom is in a line between two oxygen atoms. This hydrogen bonding tendency gets stronger as the temperature gets lower (because there is less thermal energy to shake the hydrogen bonds out of position). The ice structure is completely hydrogen bonded, and these bonds force the crystalline structure to be very "open" as freezing is achieved. Close to the freezing point, the water molecules start to arrange locally into ice-like structures. This creates some "openness" in the liquid water, which tends to decrease its density. This is opposed by the normal tendency for cooling to increase the density; it is at approximately 4 degrees Celsius that these opposing tendencies are balanced, producing the density maximum. "

[Mr_Cheesy]

Thankyou, that's brilliant!

[New Judge]

Have a look at this:

http://www.lsbu.ac.uk/water/explan2.html

[Mr_Cheesy]

Superb - just about understand the process now!