IV.+Water+expands+as+it+freezes.+Why+does+water+float?

Ben Adickes

Interestingly enough, ice is classified as a mineral.The expansion of water in the freezing process is due to hydrogen bonding. when water freezes, it causes the water molecules to "gridlock" into a specific pattern, forming crystals. this gridlock pattern forces each oxygen molecule to be surrounded with a specific number of hydrogens (i.e. 4), causing more space in between each atom than the liquid. Common ice (sticking water in a freezer), therefore, floats because it is less dense than water. There are ices that sink, but they have a different structures. a good example of this is "heavy water" which has an extra neutron, making it "heavier" than regular water. naturally, if heavy water is frozen, it makes "heavy ice", which sinks in normal water.

more on heavy water @ http://www.sno.phy.queensu.ca/sno/D2O.html