Dictionary:
sea·wa·ter (sē'wô'tər, -wŏt'ər) ![]() |
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| Britannica Concise 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.
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.
| Wikipedia: Seawater |
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 kilogram (2.2 lb) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (mostly, but not entirely, the ions of sodium chloride: Na+, Cl−). The average density of seawater at the ocean surface 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 (39 °F)) because of the salts’ added mass. The freezing point of sea water decreases with increasing salinity and is about −2 °C (28.4 °F) at 35 g/l.[1]
Contents |
| 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 unusually salty water. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater still.
The density of surface seawater ranges from about 1,020 to 1,029 kg•m−3, depending on the temperature and salinity. Deep in the ocean, under high pressure, seawater can reach a density of 1,050 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 1,500 meters/second, and varies with water temperature, salinity, and pressure.
Seawater contains more dissolved ions than all types of freshwater.[2] However, the ratios of various solutes differ dramatically. For instance; although seawater contains about 2.8 times the 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 much more quickly.[3]
| Component | Concentration (mol/kg) |
|---|---|
| H2O | 53.6 |
| Cl− | 0.546 |
| Na+ | 0.469 |
| Mg2+ | 0.0528 |
| SO2−4 | 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 after rainfall washed it out of the ground. Upon reaching the ocean, these salts concentrated as the process of evaporation (see Hydrologic cycle) removed the water. Halley noted that most lakes that don’t have ocean outlets (such as the Dead Sea and the Caspian Sea, see endorheic basin), have high salt content. Halley termed this process "continental weathering".
Halley's theory is partly correct. In addition, sodium leached out of the ocean floor when the ocean formed. The presence of salt’s other dominant ion, 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.[3] Since the ocean's formation, sodium no longer leaches from the ocean floor, but instead is captured in sedimentary layers covering the ocean bed. One theory is that plate tectonics forces salt under the continental land masses, where it slowly leaches again to the surface.
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, drinking seawater to maintain hydration is counterproductive; more water must be excreted to eliminate the salt (via urine) than the amount of water that is gained from drinking the seawater itself.[5]
This occurs because the kidney actively regulates human blood’s sodium chloride within a very narrow range around 9 g/L (0.9% by weight).[citation needed] Drinking seawater (which contains about 3.5% ions of dissolved sodium chloride) temporarily increases blood’s concentration of sodium chloride.[citation needed] This in turn signals the kidney to excrete sodium, but seawater’s sodium concentration is above the kidney’s maximum concentrating ability.[citation needed] Eventually the blood’s sodium concentration will rise to toxic levels, removing water from all cells and interfering with nerve conduction, ultimately producing fatal seizure and heart arrhythmia.[citation needed]
Survival manuals consistently advise against drinking seawater. For example, the book "Medical Aspects of Harsh Environments" (Chapter 29 - Shipboard Medicine) [2] 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 settings in rats.[6]. 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:
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 reverse osmosis. However these processes are energy intensive, and most were not available or practical during the Age of Sail.
Other land animals and marine animals such as fish, whales, and penguins can adapt to highly saline habitat. For example, the desert rat can survive by drinking seawater because its kidney can concentrate sodium far more efficiently than the human kidney.
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| Translations: Seawater |
Dansk (Danish)
n. - havvand, saltvand
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
العربيه (Arabic)
(الاسم) ماء ألبحر
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![]() | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more | |
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