Below 4°C, the hydrogen bonds between water molecules become stronger and cause the matter to expand. Because the bonds between water molecules are stronger, they are less likely to break and re-form as they do in the liquid state. As a liquid, water molecules are constantly moving (forming and breaking hydrogen bonds) resulting in less expansion.
When frozen, water molecules take a more defined shape and arrange themselves in six-sided crystalline structures. The crystalline arrangement is less dense than that of the molecules in liquid form which makes the ice less dense than the liquid water. When water freezes, volume expands by approximately 9%.
When water molecules are in the liquid state, hydrogen bonds are continuously being formed and reformed in a disordered fashion. The average amount of H-bonds a water molecule has at 25°C is 3.4 bonds. During freezing, water molecules lose energy and do not vibrate or move around as vigorously. This allows more stable hydrogen-bonds to form between water molecules, as there is less energy to break the bonds. Because of the crystal lattice structure imposed on water molecules once the freezing point is reached, the average H-bonds per water molecule is much closer to its maximum, 4 bonds.
The orderly, crystalline way in which the hydrogen-bonds form causes density to decrease because each water molecule is held away from its neighbors at a distance equal to the length of the hydrogen bonds. Thus water expands as it freezes, and ice floats atop water.
This property is crucial to life as we know it. Ice sheets on the surface of the ocean insulate the water beneath and allow organisms to live in polar regions of the world, and also prevents oceans from freezing solid.