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seawater

 
Dictionary: sea·wa·ter   (''tər, -wŏt'ər) pronunciation
 
n.

The salt water in or coming from the sea or ocean.


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Sci-Tech Encyclopedia: Seawater
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An aqueous solution of salts of a rather constant composition of elements whose presence determines the climate and makes life possible on the Earth and which constitutes the oceans, the mediterranean seas, and their embayments. The physical, chemical, biological, and geological events therein are the studies that are grouped as oceanography. Water is most often found in nature as seawater (about 98%). The rest is ice, water vapor, and fresh water. The basic properties of seawater, their distribution, the interchange of properties between sea and atmosphere or land, the transmission of energy within the sea, and the geochemical laws governing the composition of seawater and sediments are the fundamentals of oceanography. See also Hydrosphere; Oceanography.

Major constituents of seawater (salinity 35 psu)*

Amount,

Amount,

Positive ions

g/kg

Negative ions

g/kg

Sodium (Na+)

10.752

Chloride (Cl)

19.345

Magnesium (Mg2+)

 1.295

Bromide (Br)

 0.066

Potassium (K+)

 0.390

Fluoride (F)

  0.0013

Calcium (Ca2+)

 0.416

Sulfate (SO4)

 2.701

Strontium (Sr2+)

 0.013

Bicarbonate

 0.145

 (HCO3)

Boron hydroxide

 0.027

 (B(OH)3)

*Water, 965 psu; dissolved materials, 35 psu.

The major chemical constituents of seawater are cations (positive ions) and anions (negative ions) [see table]. In addition, seawater contains the suspended solids, organic substances, and dissolved gases found in all natural waters. A standard salinity of 35 practical salinity units (psu; formerly parts per thousand, or) has been assumed. While salinity does vary appreciably in oceanic waters, the fractional composition of salts is remarkably constant throughout the world's oceans. In addition to the dissolved salts, natural seawater contains particulates in the form of plankton and their detritus, sediments, and dissolved organic matter, all of which lend additional coloration beyond the blue coming from Rayleigh scattering by the water molecules. Almost every known natural substance is found in the ocean, mostly in minute concentrations. See also Scattering of electromagnetic radiation.


 

Water that makes up the oceans and seas. Seawater is a complex mixture of 96.5% water, 2.5% salts, and small amounts of other substances. Much of the world's magnesium is recovered from seawater, as are large quantities of bromine. In certain parts of the world, sodium chloride (table salt) is obtained by evaporating seawater. In addition, desalted seawater can theoretically furnish a limitless supply of drinking water, but the high processing costs are prohibitive. Large desalination plants have been built in dry areas along seacoasts in the Middle East and elsewhere to relieve shortages of fresh water.

For more information on seawater, visit Britannica.com.

 
Wikipedia: Seawater
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Sea water in the Strait of Malacca.

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every 1 kg of seawater has approximately 35 grams of dissolved salts (mostly, but not entirely, the ions of sodium chloride: Na+, Cl-). The average density of seawater at the surface of the ocean is 1.025 g/ml; seawater is denser than freshwater (which reaches a maximum density of 1.000 g/ml at a temperature of 4°C) because of the added mass of the salts. The freezing point of sea water decreases with increasing salinity and is about -2°C (28.4°F) at 35 parts per thousand. [1]

Contents

Salinity

Annual mean sea surface salinity for the World Ocean. Data from the World Ocean Atlas 2005.
Seawater composition (by mass) (salinity = 35)
Element Percent Element Percent
Oxygen 85.84 Sulfur 0.091
Hydrogen 10.82 Calcium 0.04
Chlorine 1.94 Potassium 0.04
Sodium 1.08 Bromine 0.0067
Magnesium 0.1292 Carbon 0.0028

Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water runoff from river mouths or near melting glaciers, seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and river inflow, and confined circulation result in the formation of unusually salty seawater. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater.

The density of surface seawater ranges from about 1020 to 1029 kg·m-3, depending on the temperature and salinity. Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg·m-3 or higher. Seawater pH is limited to the range 7.5 to 8.4. The speed of sound in seawater is about 1500 m·s-1, and varies with water temperature, salinity, and pressure.

Compositional differences from fresh water

Seawater is more enriched with dissolved ions of all types unlike freshwater.[2] However, the ratios of various solutes differ dramatically. For instance; although seawater is about 2.8 times more enriched, with bicarbonate than river water based on molarity, the percentage of bicarbonate in seawater as a ratio of all dissolved ions is far lower than in river water. Bicarbonate ions also constitute 48% of river water solutes, but only 0.41% of all seawater ions.[2][3] Differences like these are due to the varying residence times of seawater solutes; sodium and chlorine have very long residence times, while calcium (vital for carbonate formation) tends to precipitate out much more quickly.[3]

Geochemical explanations

Chemical composition of sea salt
Total Molal Composition of Seawater (Salinity = 35)[4]
Component Concentration (mol/kg)
H2O 53.6
Cl- 0.546
Na+ 0.469
Mg2+ 0.0528
SO42- 0.0282
Ca2+ 0.0103
K+ 0.0102
CT 0.00206
Br- 0.000844
BT 0.000416
Sr2+ 0.000091
F- 0.000068

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 billions of years, most likely as a consequence of a chemical/tectonic system which removes as much salt as is deposited; for instance, sodium and chloride sinks include evaporite deposits, pore water burial, and reactions with seafloor basalts.[5] Since the ocean's formation, 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.

Human consumption of seawater

Accidentally consuming small quantities of clean seawater is not harmful, especially if the seawater is consumed along with a larger quantity of fresh water. However, consuming seawater to maintain hydration is counterproductive; in the long run, more water must be expended to eliminate the seawater's salt (through excretion in urine) than the amount of water that is gained from drinking the seawater itself. [6]

This occurs because the amount of sodium chloride in human blood is actively regulated within a very narrow range of 9 g/L (0.9% by weight) by the kidney. Drinking seawater (which contains about 3.5% ions of dissolved sodium chloride) temporarily increases the concentration of sodium chloride in the blood. This in turn promotes sodium excretion by the kidney, but the sodium concentration of seawater is above the maximum concentrating ability of the human kidney. Eventually with further seawater intake the blood concentration of sodium will rise to toxic levels, removing water from all cells and interfering with nerve conduction ultimately giving seizures and heart arrhythmias which become fatal.

Of note, various animals adapt to harsh living conditions. For example, the desert rat is able to concentrate sodium far more efficiently than the human kidney, and therefore would be able to survive by drinking seawater.

Survival manuals consistently advise against drinking seawater. For example, the book "Medical Aspects of Harsh Environments" (Chapter 29 - Shipboard Medicine) [1] presents a summary of 163 life raft voyages. The risk of death was 39% for those who drank seawater, compared to only 3% for those who did not drink seawater. The effect of seawater intake has also been studied in laboratory setting in rats. (Etzion and Yagil; Metabolic effects in rats drinking increasing concentrations of seawater. Comp Biochem Physiol A. 1987;86(1):49-55.) [2]. This study confirmed the negative effects of drinking seawater when dehydrated.

The temptation to drink seawater has always been greatest for sailors who have expended their supply of fresh water, and are unable to capture enough rainwater for drinking. This frustration is described famously by a line from Samuel Taylor Coleridge's The Rime of the Ancient Mariner:

"Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink."

Although it is clear that a human cannot survive on seawater alone, some people claim that one can drink up to two cups a day, mixed with fresh water in a 2:3 ratio, without ill effect. The French physician Alain Bombard claimed to have survived an ocean crossing in a small raft using only seawater and other provisions harvested from the ocean, but the veracity of his findings was challenged. In Kon-Tiki, Thor Heyerdahl reported drinking seawater mixed with fresh in a 40/60% ratio. A few years later another adventurer named William Willis claimed to have drunk two cups of seawater and one cup of fresh per day for 70 days without ill effect when he lost part of his water supply. [7]

Most modern ocean-going vessels create drinkable (potable) water from seawater using desalination processes such as vacuum distillation, multi-stage flash distillation, or by the use of reverse osmosis. However these processes are energy intensive, and most were not available or practical during the Age of Sail.

See also

References

  1. ^ U.S. Office of Naval Research Ocean, Water: Temperature
  2. ^ a b Thomson Gale, "Ocean Chemical Processes". Retrieved 12/2/06.
  3. ^ a b Pinet, Paul R. (1996) Invitation to Oceanography, (St. Paul: West Publishing Company), ISBN 9780314063397, pp. 126, 134-135
  4. ^ DOE (1994) Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water; version 2, A.G. Dickson & C. Goyet, eds., ORNL/CDIAC-74. chapter 5
  5. ^ Pinet, 133.
  6. ^ Ask A Scientist - Biology Archive
  7. ^ King, Dean. (2004) Skeletons on the Zahara: a true story of survival, Back Bay Books, New York, ISBN 9780316159357, p. 74

 
Translations: Seawater
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Dansk (Danish)
n. - havvand, saltvand

Nederlands (Dutch)
zeewater

Français (French)
n. - eau de mer

Deutsch (German)
n. - Meerwasser, Seewasser

Ελληνική (Greek)
n. - θαλασσόνερα

Italiano (Italian)
acqua di mare

Português (Portuguese)
n. - água do mar (f)

Русский (Russian)
морская, забортная вода

Español (Spanish)
n. - agua de mar

Svenska (Swedish)
n. - havsvatten

中文(简体)(Chinese (Simplified))
海水

中文(繁體)(Chinese (Traditional))
n. - 海水

한국어 (Korean)
n. - 바닷물, 해수

日本語 (Japanese)
n. - 海水

العربيه (Arabic)
‏(الاسم) ماء ألبحر‏

עברית (Hebrew)
n. - ‮מי-ים‬


 
 

 

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Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Seawater" Read more
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