Why is sea water salty unlike river water?



The oceans have been around a very long time, so some of the salts were added to the water at a time when gases and lava were spewing from increased volcanic activity. The carbon dioxide dissolved in water from the atmosphere forms weak carbonic acid which dissolves minerals. When these minerals dissolve, they form ions, which make the water salty. While water evaporates from the ocean, the salt gets left behind. Also, rivers drain into the oceans, bringing in additional ions from rock that was eroded by rainwater and streams.

The saltiness of the ocean, or its salinity, is fairly stable at about 35 parts per thousand. To give you a sense of how much salt that is, it is estimated that if you took all the salt out of the ocean and spread it over the land, the salt would form a layer more than 500 feet (166 m) deep! You might think the ocean would become increasingly salty over time, but part of the reason it does not is because many of the ions in the ocean are taken in by the organisms that live in the ocean. Another factor may be the formation of new minerals.

The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid (which forms from carbon dioxide and water). The rain erodes the rock and the acid breaks down the rocks and carries it along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water.

Others are not used up and are left for long periods of time where their concentrations increase over time. The two ions that are present most often in seawater are are chloride and sodium.

These two make up over 90 per cent of all dissolved ions in seawater.When sea water is warmed and evaporates into the atmosphere, the salt that it contains is left behind. When this water precipitates over land in the form of rain or snow, it feeds freshwater bodies such as rivers and lakes.When fresh water moves over rocks, it dissolves a little of the rock, thereby picking up salt.

The rivers then carry this salt back out into the ocean.In general, fresh water has only a few milligrams of salt per litre.The ocean is thousands of times more salty than that: ocean water contains about 3500 milligrams of salt per litre.


Associate professor

S.N. College, Kollam , Kerala


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