Behavior of water and ice

[Poetman]

Liquids are normally less dense than the solids formed from them on cooling - one assumes that is due to the close packing of atoms or

molecules in a crystalline state. But water is different, it expands as it forms ice, causing pipes to fracture. It must be this expansion which allows the ice to float in water.

Why does water do this when other liquids (to my knowledge) don't ? Explain please.

[weiming1998]

This should explain it: http://www.word-detective.com/howcome/waterexpand.html

 

Basically, liquid water flows past each other, forming hydrogen bonds with the surrounding water molecules. These hydrogen bonds break and reform. But when water is frozen, it crystallizes and bonds to surrounding molecules in a rigid, hexagonal ring-like structure, with empty space between the rings. The hydrogen bonds between the water molecules can no longer break and reform easily. This cause the water to expand as it freezes.

 

An analogy is styrofoam. When styrofoam is melted the rigid polymer chains that traps the air between it collapses due to liquefaction, air is released and the previous "empty" spaces are filled, and it becomes much more dense than the original solid styrofoam. Same with water, except it occurs on a microscopic scale, and the space between the water molecules reform once it is frozen again.

[Ophiolite]

Water molecules tend to link together in loosely bound structures through hydrogen bonding. The number of molecules so linking tends to increase with decreasing temperature. The linked molecule form loose structures that take up more space than unbound molecules. At around 4 deg centigrade this trend 'overwhelms' the normal reduction in volume caused by lower temperature. Further reduction in temperature leads to expansion. Wikipedia likely has some good articles on this.