Frozen water is less dense than liquid water

             As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered a mineral. It possesses a regular crystalline structure based on the molecule of water, which consists of a single oxygen atom covalently bonded to two hydrogen atoms, or H-O-H. However, many of the physical properties of water and ice are controlled by the formation of hydrogen bonds between adjacent oxygen and hydrogen atoms. It is a weak bond, but is critical in controlling the structure of both water and ice. Ice is the only known non-metallic substance to expand when it freezes. The density of ice is 0.9167 g/cm³ at 0°C, whereas water has a density of 0.9998 g/cm³ at the same temperature. When cooled from room temperature, liquid water becomes increasingly dense, just like other substances. But at approximately 4 °C, water reaches its maximum density. As it is cooled further under ambient conditions, it expands to become less dense. This unusual negative thermal expansion is attributed to strong, orientation-dependent and intermolecular interactions. The solid form of most substances is denser than the liquid phase; thus, a block of the solid will sink in the liquid. But, by contrast, a block of common ice floats in liquid water because ice is less dense than liquid water. Upon freezing, the density of ice decreases by about 9%. The reason for this is the 'cooling' of intermolecular vibrations allowing the molecules to form steady hydrogen bonds with their neighbors and thereby gradually locking into positions reminiscent of the hexagonal packing achieved upon freezing to ice . While the hydrogen bonds are shorter in the crystal than in the liquid, this locking effect reduces the average coordination number of molecules as the liquid approaches nucleation. One of the many benefits of this unique property of water can be seen in winter. Water freezes on the top of lakes first and insulates the layers below from further cooling and freezing, thus protecting and allowing aquatic life forms to thrive in the water below the ice. If ice was denser than liquid water, then lakes would freeze from the bottom up and be frozen solid during winter, eliminating fish and all other aquatic life forms.