Seawater is the salty water that occurs in seas and oceans. It is distinguished from pure water by the large number of dissolved substances in it, such as inorganic solids, organic substances and gases. In addition, seawater also contains suspended matter such as silt particles and plankton.
Seawater contains various salts in a dissolved state and is therefore unsuitable as drinking water. The salinity or salinity is expressed in permilles (‰). The following amounts of salts occur on average in 1 liter of seawater:
- 24 grams of sodium chloride (NaCl)
- 5 grams of magnesium chloride (MgCl2)
- 4 grams sodium sulfate (Na2SO4)
- 0.8 grams of magnesium bromide (MgBr2)
- 0.7 grams of calcium chloride (CaCl2)
The amount of salts is therefore on average 34.5 grams per liter of seawater, or 34.5‰ (m/m). A bucket with a capacity of 10 liters of seawater therefore contains approximately two full coffee cups of sea salt. However, the salinity is not the same everywhere in the gallery. The highest salinities occur in the subtropical belts, namely 36.3‰. It is less along the equator, despite the large evaporation: 34.3‰. This is caused by the heavy rainfall. The Mediterranean Sea has a higher salinity, increasing from west to east, with about 39‰ in the eastern part. The Red Sea has the highest salinity for open seas at 41 to 42‰.
In some closed seas, the salinity differs greatly from that of the open seas. For example, the Salinity of the Dead Sea at 337‰ is about 10 times higher than that of ocean water, while the Caspian Sea with an average salinity of 12‰ is much less salty than ‘normal’ seas. At high latitudes, salinity is significantly lower, especially near the poles. This is due to the low evaporation and the large amount of melt water. At the North Pole also because of the many fresh water that is supplied by the great Siberian rivers. The Baltic Sea is also less salty, less than 25‰, because many rivers flow into it and there is little evaporation. The north of the Gulf of Bothnia can even be called sweet.
In the North Sea, the salinity fluctuates between 34‰ and 35‰. Just below the coasts – where large rivers bring fresh water into the sea – the salinity is lower. Off the Belgian coast, the salinity from west to east is about 34‰ to 31‰. It is lower than 30‰ on the Dutch coast and in the German Bight.
Seawater is a good electrical conductor because of the salt. The higher the temperature or salinity, the lower the resistance. At a salt content of 35‰ and a temperature of 15 °C, the specific conductivity is approximately 42 mS/cm.
Some organisms are very tolerant of salinity. Such animals or plants are called euryhalines or halophytes in plants. An example of a euryhaline fish is the flounder (fish) (Platichthysflesus), which is found both in the high seas and in (rather sweet) estuaries.
If organisms are strictly bound to a certain salinity, they are called stenohaline. Many echinoderms are stenohaline and rarely or never occur in brackish or fresh water.
Human salt tolerance
Normally a sip of seawater is not harmful, and only very salty. But for someone who is without drinking water at sea for a long time, drinking sea water is very harmful. For castaways and others who are at sea for long periods without drinking water, it can be tempting to drink sea water. However, when a person has not drunk fresh water for some time, the human body becomes dehydrated. When seawater is drunk, the body is no longer able to flush the salt out of the body. This can cause salt poisoning, which can cause delusions, among other things. By drinking seawater, castaways quickly reduce their chances of survival.
It is possible to convert seawater into potable water by means of reverse osmosis. The water is then forced through a semi-permeable fleece under high pressure. The osmotic value of seawater is approximately 30 bar and is therefore quite high. A disadvantage of the method – in addition to the high energy consumption – is that the desalination is never completely perfect. When desalinated water is used for irrigation, it can lead to a slow build-up of salt in the fields.