<P> An unusual property of ice frozen at atmospheric pressure is that the solid is approximately 8.3% less dense than liquid water . The density of ice is 0.9167 g / cm at 0 ° C, whereas water has a density of 0.9998 g / cm at the same temperature . Liquid water is densest, essentially 1.00 g / cm, at 4 ° C and becomes less dense as the water molecules begin to form the hexagonal crystals of ice as the freezing point is reached . This is due to hydrogen bonding dominating the intermolecular forces, which results in a packing of molecules less compact in the solid . Density of ice increases slightly with decreasing temperature and has a value of 0.9340 g / cm at − 180 ° C (93 K). </P> <P> When water freezes, it increases in volume (about 9% for fresh water). The effect of expansion during freezing can be dramatic, and ice expansion is a basic cause of freeze - thaw weathering of rock in nature and damage to building foundations and roadways from frost heaving . It is also a common cause of the flooding of houses when water pipes burst due to the pressure of expanding water when it freezes . </P> <P> The result of this process is that ice (in its most common form) floats on liquid water, which is an important feature in Earth's biosphere . It has been argued that without this property, natural bodies of water would freeze, in some cases permanently, from the bottom up, resulting in a loss of bottom - dependent animal and plant life in fresh and sea water . Sufficiently thin ice sheets allow light to pass through while protecting the underside from short - term weather extremes such as wind chill . This creates a sheltered environment for bacterial and algal colonies . When sea water freezes, the ice is riddled with brine - filled channels which sustain sympagic organisms such as bacteria, algae, copepods and annelids, which in turn provide food for animals such as krill and specialised fish like the bald notothen, fed upon in turn by larger animals such as emperor penguins and minke whales . </P> <P> When ice melts, it absorbs as much energy as it would take to heat an equivalent mass of water by 80 ° C. During the melting process, the temperature remains constant at 0 ° C. While melting, any energy added breaks the hydrogen bonds between ice (water) molecules . Energy becomes available to increase the thermal energy (temperature) only after enough hydrogen bonds are broken that the ice can be considered liquid water . The amount of energy consumed in breaking hydrogen bonds in the transition from ice to water is known as the heat of fusion . </P>

Importance of low density of ice in ecosystem
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