Ice is less dense than water due to the unique arrangement of water molecules when they freeze. When water is in its liquid form, the molecules are in constant motion, and they are close together, allowing for a relatively high density.
However, as water cools and begins to freeze, the molecules start to slow down and form hydrogen bonds with each other. These hydrogen bonds create a crystalline structure in ice, where the molecules are spaced further apart than in liquid water. This structure has more open spaces, which decreases the overall density of the ice compared to liquid water.
As a result, ice floats on water because the density of ice is less than that of liquid water. This phenomenon is quite unusual since most substances become denser as they transition from a liquid to a solid state. The peculiar behavior of water is essential for aquatic life, as it prevents lakes and rivers from freezing solid, allowing life to thrive underneath the ice.