Seawater

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 599 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (predominantly sodium (Na+) and chloride (Cl−) ions). Average density at the surface is 1.025 kg/l. Seawater is denser than both fresh water and pure water (density 1.0 kg/l at the surface) because the dissolved salts increase the mass by a larger proportion than the volume. The freezing point of seawater decreases as salt concentration increases. At 3.5% salinity, it freezes at about −2 °C (28 °F).

Composition

The most abundant dissolved ions in seawater are sodium, chloride, magnesium, sulfate and calcium. Its salinity varies with temperature, evaporation, rain, river inflow, and ice melt.

Physical properties

Seawater contains more dissolved ions than all types of freshwater. However, the ratios of solutes differ dramatically. For instance, although seawater contains about 2.8 times more bicarbonate than river water, 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 0.14% of all seawater ions. Differences like these are due to the varying residence times of seawater solutes; sodium and chloride have very long residence times, while calcium (to a lesser degree) tends to precipitate much more quickly.

Salinity

Salinity is the total amount of solid material in grams contained in one kilogram of sea water and is described as a ratio and is unitless. Salinity was traditionally measured by titration method but now it is measured by conductivity, refractive index, or by density.

Desalination

Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in soil desalination, which is an issue for agriculture.

See also

• Oceanography
• Salinity
• Desalination
• Brackish water
• Brine

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