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Seawater
Seawater
A solution of many different substances dissolved in water. The majority of the dissolved material is salt, but seawater contains many other substances that are vital
for ocean life, including oxygen, nitrogen, and carbon.
The sea contains an average
of 35g of salt in each kilogram
of water. However, near river mouths, near the equator, and in high latitudes rainfall and fresh water dilute the sea slightly. In contrast, in the subtropics the sea is saltier than average due to the large amount of water that evaporates under the hot, sunny skies. (See also: Desalinization.)
Most of the dissolved minerals in seawater are brought in by rivers. Rivers also carry large amounts of sediment. The sediment gradually
settles out of the seawater. More minerals are introduced into the sea through hot springs in the seabed. These springs are known as hydrothermal vents.
Many of the minerals are used to make the skeletons of sea creatures. When these creatures die, their skeletons sink to the seafloor and consolidate as rock.
It is believed that the Earth’s oceans began in the air. Very early in the history of the Earth the planet condensed from the cold dust of space, then heated up and became molten, expelling gases that included water vapor.
It took hundreds of millions of years before the temperature of the air cooled below 100°C. But once this temperature had been reached, water condensed out of the air to form a very hot ocean.
 Seawater—Salt is one of the valuable minerals that can be recovered from seawater by evaporation.
It was also a very acid ocean,
and the acids in the water reacted with the rocks of the Earth’s
crust. It was during this time of chemical reactions that the chloride in hydrochloric acid formed a compound with the sodium in the crustal rocks to produce the salt (sodium chloride) that is now found in ocean water. So, even at this early stage the ocean became salty. Since this time some of the salt has been turned into rock salt, and so the modern ocean is only about half as salty as the early ocean.
The main chemical parts of seawater are chloride, sulfate, bicarbonate, bromide, fluoride, sodium, magnesium, calcium,
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