The physical state of water, whether liquid, gas or solid, is determined by the speed at which hydrogen bonds are being formed and broken. Heat increases that speed; hence weak hydrogen bonds cannot hold molecules together as they move faster.
The thermal status of water in a liquid state is such that hydrogen bonds are being broken as fast as they form. At low temperatures, the tetrahedral arrangement is almost perfect; when water freezes, the tetrahedral arrangement becomes an almost perfect lattice with considerable open space between ice crystals and thus, a decrease in density. This is the reason why ice floats on water body.
As the temperature of frozen water is increased, this molecular arrangement becomes looser and more diffuse, resulting in random packing due to continuous breaking and reforming of hydrogen bonds and contraction of molecules. The higher the temperature rises, the more diffuse the pattern becomes, until the hydrogen bonds and whole structure break, down and as a result, water melts.
Upon melting, water contracts and its density increases up to a temperature of 3’98’C. Beyond this point the loose arrangement of the molecules means a reduction in density again. The existence of this point of maximum density at 398?C or 4°C is of fundamental significance to aquatic organisms (Smith, 1977).
However, the behaviour of seawater is somewhat different. The density (or mass per unit, volume) of sea water (salinity of 24 7%, and higher) or, rather, its specific gravity relative to that of an equal volume of pure water (sp gr = 1) at atmospheric pressure, is correlated with salinity.
At 0°C, the density of sea water with a salinity of 35% is T028 the lower its temperature, the greater the density of sea water, the higher the temperature, the lower the density. No definite freezing point exists for sea water. Ice crystals begin to form at a point on the temperature scale that varies with salinity. As pure water freezes out, the remaining unfrozen water becomes higher in salinity and lower in its freezing point until finally a solid block of ice crystals and salt is formed.