Oceans are salty but lakes and rivers aren't: Why is it so?
It’s a known fact that two-third of the earth’s surface is replete with water and a staggering 97 per cent of it comprises of salty seawater. So, only three per cent of water in this planet is fresh, out of which 2 per cent is trapped in soil, glaciers and ice caps. This leaves only one per cent fresh water in lakes and rivers. However, the nature of river and lake water plays a significant role behind the salinity of sea water.
Decoded: Salinity of sea water
High levels of sodium chloride make sea water salty. The salt build-up in oceans has been occurring through billions of years. That is why they have 300 times more dissolved salts than the fresh river water. In fact, one litre of sea water has 35 grams of dissolved salt.
But where do oceans get the salt from? Well, this is how it goes. Rainwater, which is slightly acidic in nature due to the presence of carbon dioxide and sulphur dioxide, picks up mineral salts from rocks including sodium and chloride. It flows from the ground to rivers and streams, which make their way to oceans, depositing the dissolved salts therein. Seas get salty because the process of salt build-up has been continuing for billions of years.
Sodium and chloride accumulate in oceans because the only way water leaves it is through evaporation, a process that doesn’t take the salt away. Hot vents of the deep ocean floor and volcanoes on the land are also the sources of sea water salt. Some of it is, however, removed by the plants and animals living in the sea and some is deposited as sediment on the ocean bed.
The salinity of sea water varies, depending on climatic conditions. Oceans of warm, tropical areas tend to be saltier due increased rate of evaporation. On the other hand, seas towards the north and south poles have their water diluted with melting ice which decreases the quantity of salt.
Why doesn’t river water taste salty?
River water carries very low amounts of dissolved salts unlike ocean water. One litre of fresh river water contains only .5 grams of salt. That is why its salinity is very low. The low concentration of salt can be attributed to the fact that there is no build-up process. Water from river always flows to the sea.
Why don’t lakes have salt in their water?
Lakes draw their water from streams and rivers and they are also in touch with the ground. But still lake water doesn’t taste salty. Why is it so? Well, some of the lakes, like the Great Salt Lake in Uttah, Sambhar Lake in Rajasthan, Chilika Lake in Odhisa and The Dead Sea in Jordan are salty. This is because their water contains sodium and chloride ions while the others don’t. Most lakes are filled with minerals other than sodium and chloride, which don’t lend a salty taste to their water. There is another reason behind the lack of salinity in lakes. There is a constant outflow of water from lakes, which takes away the low concentration of minerals in their water.