Escola Secundária de Loulé
Due to its molecular structure and its hydrogen bonds water presents some special properties. An unusual feature of water is its anomalous expansion. It contracts with the drop in temperature, but at 4°C begins to expand, returning to his contract after solidification.
Life under frozen water. Credit: Wikipedia(http://pt.wikipedia.org/wiki/Imagem:H2O.jpg)
That explains why the water first freezes on the surface because the water that reaches a temperature of 0ºC becomes less dense than water at 4°C, which mean it stays at the surface. This phenomenon is also important for the maintenance of life in cold water, because makes the water at 4°C is at the bottom and holds more heat for the creatures that live there. The lower density is due to the holes in the three-dimensional hexagonal structure due to hydrogen bonds typical of ice.
Ice three-dimensional structure. Credit:NYU (http://www.nyu.edu/pages/mathmol/textbook/hbond_ice.gif))
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