Hydrogen+Bonding+in+ice

Ice is one of the only liquids that is less dense when solid than liquid. This is a cool fact, but what's its significance? As ice freezes and floats to the surface it insulates the water below; keeping it from freezing. This is not only good for marine animals, it's good for humans too. If ice was more dense than water, the ice would fall to the bottom of the water and would in turn not stay frozen during the summer. Before long, entire bodies would be frozen all year round with the exception of a thin layer on the surface. No water would be available for terrestrial life and we would all die.

Why does ice behave in this way? Since water contains hydrogen, it is capable of forming hydrogen bonds. Hydrogen bonding is what allows ice to have this unique property. Hydrogen bonds allow a more open network than what would normally exist because of the way that the bonds are set up and the fact that they repel each other. As a result, even though we expect solid water (ice) to be more dense than liquid water, the opposite is true and ice floats on water.