This was my ACE for last term :)
Objects that are less dense float on liquid that is denser. When water freezes, it actually becomes les dense. Ice is 9% less dense than water. The water exerts a normal force on the ice, so it remains afloat. Unlike other substances which are denser in their solid forms, water is different because of Hydrogen Bonding. Water is a compound, and each water molecule consists of one oxygen atom and two hydrogen atoms. Water molecules are attracted by Hydrogen Bonds between the positively-charged hydrogen atoms and negatively-charged oxygen atoms. When water cools to 4 degrees Celcius, these Hydrogen bonds will alter to hold the oxygen atoms apart, which forms a crystal lattice, ice.
The floating of ice have allowed aquatic freshwater organisms to survive during the winter when the top layer of the river have froze. Unfortunately, it has also caused RMS Titanic to sink to the ocean depths...
More on Ice Floating: http://chemistry.about.com/od/chemistryfaqs/f/icefloats.htm
http://www.chemistryquestion.com/English/Questions/ChemistryInDailyLife/5c_Water_Ice.html
Video on Ice Floating: Enjoy! :D
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