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Seawater

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Annual mean sea surface temperature for the World Ocean. Data from the
   World Ocean Atlas 2001.
   Enlarge
   Annual mean sea surface temperature for the World Ocean. Data from the
   World Ocean Atlas 2001.

   Seawater is water from a sea or ocean. On average, seawater in the
   world's oceans has a salinity of ~3.5%. This means that for every 1
   litre (1000 mL) of seawater there are 35 grams of salts (mostly, but
   not entirely, sodium chloride) dissolved in it. This can be expressed
   as 0.6 M NaCl or 0.6  mol·L^-1 (if the salinity were due entirely to
   NaCl, which it is not).

   Although a vast majority of seawater is found in oceans with salinity
   around the 3.5 %, seawater is not uniformly saline throughout the
   world. The planet's freshest (least saline) sea water is in the eastern
   parts of Gulf of Finland and in the northern end of Gulf of Bothnia,
   both part of the Baltic Sea. The most saline open sea is the Red Sea,
   where high temperatures and confined circulation result in high rates
   of surface evaporation and there is little fresh inflow from rivers.
   The salinity in isolated seas and salt-water lakes (for example, the
   Dead Sea) can be considerably greater.

   The density of seawater is between 1020 and 1030 kg·m^-3. Seawater pH
   is limited to the range 7.5 to 8.4. The speed of sound in seawater is
   about 1500 m·s^-1.

Geochemical explanations

   Chemical composition of sea salt
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   Chemical composition of sea salt

   CAPTION: Elemental composition of Earth's ocean water (by mass)

    Element  Percent  Element  Percent
   Oxygen    87.7    Sulfur    0.0885
   Hydrogen  10.8    Calcium   0.04
   Chlorine  1.9     Potassium 0.0380
   Sodium    1.05    Bromine   0.0065
   Magnesium 0.1350  Carbon    0.0026

   Scientific theories behind the origins of sea salt started with Sir
   Edmond Halley in 1715, who proposed that salt and other minerals were
   carried into the sea by rivers, having been leached out of the ground
   by rainfall runoff. Upon reaching the ocean, these salts would be
   retained and concentrated as the process of evaporation (see Hydrologic
   cycle) removed the water. Halley noted that of the small number of
   lakes in the world without ocean outlets (such as the Dead Sea and the
   Caspian Sea, see endorheic basin), most have high salt content. Halley
   termed this process "continental weathering".

   Halley's theory is partly correct. In addition, sodium was leached out
   of the ocean floor when the oceans first formed. The presence of the
   other dominant ion of salt, chloride, results from "outgassing" of
   chloride (as hydrochloric acid) with other gases from Earth's interior
   via volcanos and hydrothermal vents. The sodium and chloride ions
   subsequently became the most abundant constituents of sea salt.

   Ocean salinity has been stable for millions of years, most likely as a
   consequence of a chemical/tectonic system which recycles the salt.
   Since the ocean's creation, sodium is no longer leached out of the
   ocean floor, but instead is captured in sedimentary layers covering the
   bed of the ocean. One theory is that plate tectonics result in salt
   being forced under the continental land masses, where it is again
   slowly leached to the surface.

Potability

   CAPTION: Total Molal Composition of Seawater (Salinity = 35)

   Component Concentration (mol . kg^-1)
   H[2]O     53.6
   Cl^-      0.546
   Na^+      0.469
   Mg^2+     0.0528
   SO[4]^2-  0.0283
   Ca^2+     0.0103
   K^+       0.0102
   C[T]      0.00206
   Br^-      0.000844
   B[T]      0.000416
   Sr^2+     0.000091
   F^-       0.000068

   Even on a ship or island in the middle of the ocean, there can be a
   "shortage of water" meaning, of course, a shortage of fresh water. This
   is described famously by a line from Samuel Taylor Coleridge's The Rime
   of the Ancient Mariner:

          "Water, water, every where
          Nor any drop to drink."

   Seawater can be turned into drinkable ( potable) water by one of a
   number of desalination processes, or by diluting it with fresh water to
   reduce the salinity. Otherwise, it should not be drunk because of its
   high dissolved mineral content. In the long run, more water must be
   expended to eliminate these minerals (through excretion in urine) than
   is gained from drinking the seawater itself.

Seawater for flushing toilet

   Hong Kong has an extensive use of seawater for flushing toilets
   citywide. More than 90% of its toilets are flushed by seawater as a
   means of conserving fresh water resources. The development of this
   approach was started in the 1960s and 1970s when water shortages became
   a severe problem as the population of the (then) British colony grew.
   This is unusual because saline water cannot be treated (in a waste
   water treatment plant) by the usual methods.
   Retrieved from " http://en.wikipedia.org/wiki/Seawater"
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