l.2 Physical and Chemical Properties of Sea Water
5
waste products. Because of its unusual physical properties, a quite different
set of environmental conditions is presented to amoebae and fish that both live
in the same waters (Franks, 1972; Gordon et al., 1992).
The principal sea water state parameter is density, or mass per unit volume.
The density of water controls many fundamental processes in the ocean, e.g. the
hydrostatic equilibrium or motion of water particles and propagation of internal
waves, turbulence and mixing in water column, heat transfer, concentration
of plankton and sediment transport, locomotion of marine species and many
others. Even small alterations in the density of sea water result in great changes
in water flow and its thermal and chemical status.
Density is normally symbolized by p and in the SI system it is expressed in
kilograms per cubic meter (kg/m 3 ). For example, the density of pure water
is approximately 1000 kg/m 3 being 770 to 890 times that of air at sea level.
Detailed information on unit systems used in the physics of fluids is given in
Appendix B.
Unlike measuring salinity, temperature and pressure, there are no practical
methods of measuring the density of sea water in situ. The density of sea water
is usually determined through its dependence on pressure, p, temperature, T,
and salinity,S. Pressure, p, has an insignificant effect on the density of water
for most applications, unless one is dealing with water at great depths within
the ocean. For example, at a depth of 1 km the density of water is only 0.5%
greater than at the surface, and at the deepest point in the earth's oceans,
the Marianas Trench, water is about 6% denser than water at the surface,
assuming that the temperature and salinity are constant (Denny, 1993). The
density is much more dependent on temperature, generally decreasing as the
temperature increases. However, the density of freshwater is not a monotonic
function of temperature; water density reaches a maximum at temperature of
3.98°C under normal atmospheric pressure (see Fig. l.2). Normal atmospheric
pressure at sea level, called the normal atmosphere, is defined to be l.01325
x 10 5 Pa at 15° C, in which 1 Pa is equal to 1 N/m 2 (see Appendix B). This
anomalous dependence of the density of freshwater on temperature is a result
of the specific molecular structure of water (Franks, 1972; Harvey, 1985; Dera,
1992).
Salinity is another factor which influences water density. Away from coasts,
the salinity of ocean water varies from 32 to 37 ppm (parts per million). The
variations in salinity result from the differences in th~ relative rates of precipitation and evaporation from the surface of the ocean. When the temperature,
T, and pressure, p, are constant, density of sea water increases with salinity
and a relative change in the density of sea water is roughly proportional to
a change of salinity. Ocean water does not demonstrate the anomalous thermal expansion of freshwater. At constant pressure and at salinity greater than
rv 24.7ppm, the density of sea water increases monotonically with decreasing
temperature right down to the freezing point, as shown in Fig. l.2. At O°C,
freshwater turns into ice, and its density abruptly decreases from 999 kg/m 3
5
waste products. Because of its unusual physical properties, a quite different
set of environmental conditions is presented to amoebae and fish that both live
in the same waters (Franks, 1972; Gordon et al., 1992).
The principal sea water state parameter is density, or mass per unit volume.
The density of water controls many fundamental processes in the ocean, e.g. the
hydrostatic equilibrium or motion of water particles and propagation of internal
waves, turbulence and mixing in water column, heat transfer, concentration
of plankton and sediment transport, locomotion of marine species and many
others. Even small alterations in the density of sea water result in great changes
in water flow and its thermal and chemical status.
Density is normally symbolized by p and in the SI system it is expressed in
kilograms per cubic meter (kg/m 3 ). For example, the density of pure water
is approximately 1000 kg/m 3 being 770 to 890 times that of air at sea level.
Detailed information on unit systems used in the physics of fluids is given in
Appendix B.
Unlike measuring salinity, temperature and pressure, there are no practical
methods of measuring the density of sea water in situ. The density of sea water
is usually determined through its dependence on pressure, p, temperature, T,
and salinity,S. Pressure, p, has an insignificant effect on the density of water
for most applications, unless one is dealing with water at great depths within
the ocean. For example, at a depth of 1 km the density of water is only 0.5%
greater than at the surface, and at the deepest point in the earth's oceans,
the Marianas Trench, water is about 6% denser than water at the surface,
assuming that the temperature and salinity are constant (Denny, 1993). The
density is much more dependent on temperature, generally decreasing as the
temperature increases. However, the density of freshwater is not a monotonic
function of temperature; water density reaches a maximum at temperature of
3.98°C under normal atmospheric pressure (see Fig. l.2). Normal atmospheric
pressure at sea level, called the normal atmosphere, is defined to be l.01325
x 10 5 Pa at 15° C, in which 1 Pa is equal to 1 N/m 2 (see Appendix B). This
anomalous dependence of the density of freshwater on temperature is a result
of the specific molecular structure of water (Franks, 1972; Harvey, 1985; Dera,
1992).
Salinity is another factor which influences water density. Away from coasts,
the salinity of ocean water varies from 32 to 37 ppm (parts per million). The
variations in salinity result from the differences in th~ relative rates of precipitation and evaporation from the surface of the ocean. When the temperature,
T, and pressure, p, are constant, density of sea water increases with salinity
and a relative change in the density of sea water is roughly proportional to
a change of salinity. Ocean water does not demonstrate the anomalous thermal expansion of freshwater. At constant pressure and at salinity greater than
rv 24.7ppm, the density of sea water increases monotonically with decreasing
temperature right down to the freezing point, as shown in Fig. l.2. At O°C,
freshwater turns into ice, and its density abruptly decreases from 999 kg/m 3