<P> Seawater, or salt water, 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 litre 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 4 ° C (39 ° F)) 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 typical salinity, it freezes at about − 2 ° C (28 ° F). The coldest seawater ever recorded (in a liquid state) was in 2010, in a stream under an Antarctic glacier, and measured − 2.6 ° C (27.3 ° F). Seawater pH is typically limited to a range between 7.5 and 8.4 . However, there is no universally accepted reference pH - scale for seawater and the difference between measurements based on different reference scales may be up to 0.14 units . </P> <P> Although the vast majority of seawater has a salinity of between 31 g / kg and 38 g / kg, that is 3.1 - 3.8%, seawater is not uniformly saline throughout the world . Where mixing occurs with fresh water runoff from river mouths, near melting glaciers or vast amounts of precipitation (e.g. Monsoon), seawater can be substantially less saline . The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and low river run - off, and confined circulation result in unusually salty water . The salinity in isolated bodies of water can be considerably greater still - about ten times higher in the case of the Dead Sea . Historically, several salinity scales were used to approximate the absolute salinity of seawater . A popular scale was the "Practical Salinity Scale" where salinity was measured in "practical salinity units (psu)". The current standard for salinity is the "Reference Salinity" scale with the salinity expressed in units of "g / kg". </P> <P> The density of surface seawater ranges from about 1020 to 1029 kg / m, depending on the temperature and salinity . At a temperature of 25 ° C, salinity of 35 g / kg and 1 atm pressure, the density of seawater is 1023.6 kg / m . Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg / m or higher . The density of seawater also changes with salinity . Brines generated by seawater desalination plants can have salinities up to 120 g / kg . The density of typical seawater brine of 120 g / kg salinity at 25 ° C and atmospheric pressure is 1088 kg / m . Seawater pH is limited to the range 7.5 to 8.4 . The speed of sound in seawater is about 1,500 m / s (whereas speed of sound is usually around 330 m / s in air at roughly 1000hPa pressure, 1 atmosphere), and varies with water temperature, salinity, and pressure . The thermal conductivity of seawater is 0.6 W / mK at 25 ° C and a salinity of 35 g / kg . The thermal conductivity decreases with increasing salinity and increases with increasing temperature . </P> <Table> Seawater elemental composition (salinity = 3.5%) <Tr> <Th> Element </Th> <Th> Percent by mass </Th> </Tr> <Tr> <Td> Oxygen </Td> <Td> 85.84 </Td> </Tr> <Tr> <Td> Hydrogen </Td> <Td> 10.82 </Td> </Tr> <Tr> <Td> Chloride </Td> <Td> 1.94 </Td> </Tr> <Tr> <Td> Sodium </Td> <Td> 1.08 </Td> </Tr> <Tr> <Td> Magnesium </Td> <Td> 0.1292 </Td> </Tr> <Tr> <Td> Sulphur </Td> <Td> 0.091 </Td> </Tr> <Tr> <Td> Calcium </Td> <Td> 0.04 </Td> </Tr> <Tr> <Td> Potassium </Td> <Td> 0.04 </Td> </Tr> <Tr> <Td> Bromide </Td> <Td> 0.0067 </Td> </Tr> <Tr> <Td> Carbon </Td> <Td> 0.0028 </Td> </Tr> <Tr> <Td> Vanadium </Td> <Td> 1.5 × 10--3.3 × 10 </Td> </Tr> </Table>

Properties of ocean water because it contains salt
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