(Lansing State Journal, April 6, 1994)
When water is a liquid, the molecules mix around very closely to one another. Each molecule is attracted to other by forces called hydrogen bonds. These bonds keep the molecules close together but not permanently fixed in one place. As one molecule moves past another it will break old hydrogen bonds and form new ones, with different water molecules.
As water cools, the molecules slow down, and can get closer together. At 4 degrees Celsius (about 39 degrees Fahrenheit), water molecules are as closely packed as they can get. When water is cooled below 4 degrees Celsius, the individual molecules start to arrange themselves into a more stable form. At 0 degrees Celsius (32 degrees Fahrenheit) and normal atmospheric pressure, water will form the stable solid we call ice.
Ice, like most other pure solids, has a crystalline structure. This means that the atoms are organized in a simple repeating structure, such as a cube or a tetrahedron. The crystalline structure of ice is a repeating arrangement of eight molecules of water. This arrangement is actually less dense than liquid water at 4 degrees Celsius! There are more molecules in the same amount of space in the cold liquid than there is in the solid form.
This is a very unique property of water, as most chemicals have solid forms that are denser than their liquid forms. The arrangement of hydrogen and oxygen atoms in water results in this and other special chemical properties.