How is it possible for ice to float on top of liquid water, instead of sinking to the bottom? The answer lies in the density of each substance.
If substance is less dense than the water, it floats.
According to Chemistry.about.com water reaches its maximum density at 4°C (40°F). As it cools further and freezes into ice, it actually becomes less dense. On the other hand, most substances are most dense in their solid (frozen) state than in their liquid state. Water is different because of hydrogen bonding.