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Why do we have both salt water and fresh water on the Earth?

71% of the Earth’s surface is covered with water, its water reserves roughly correspond to 1.4 billion km3, which equals to 0.02% of the planet’s total weight. This volume is so enormous that a 2,700-metre thick shell could be formed from it around the planet if evenly distributed.

A drop of fresh with some salt

97% of the water reserves come from the salt water of oceans and seas, while the remaining part is fresh water. A large part (roughly 80%) of this 3% fresh water lies in ice sheets; this means that only a rather low amount of fresh water is at our disposal to use directly.

Most of the fresh water appears as groundwater below the surface or as rivers and lakes on the surface. Finally, a smaller part of the fresh water moves in the air as clouds, fog or water vapour.

The salt of life

Salt water, which makes up the largest part of the water reserves, has an average salt content of 3.47%. It is mostly composed of sodium chloride and to a lesser extent of magnesium chloride, magnesium sulfate and other substances. If we measured the salinity of the world’s oceans, the result would be five trillion tons of salt. But how, and more importantly, when did this huge amount of salt appear and why cannot we find any in fresh waters?

Everything needs some salt in it!

It is a golden rule to put a touch of salt in everything we cook or bake. No matter whether cocoa, custard or fruit soup, it makes food tastier.

Though we differentiate between fresh and salt water, actually fresh water is also salty, but only to a minor extent: compared to the 3.47% salt content of sea water, fresh water contains only 0.05% of salt. This is so negligible that we cannot sense either its taste or its effect.

And it kept raining and raining

In the time when oceans were formed, that is, a few billion years ago, there was much more carbon dioxide in the atmosphere than oxygen. At that time, there was a lot of rain, and therefore acid rain, making the seas and oceans developing from this huge amount of rainwater slightly acidic. And this acidity dissolved sodium, chlorine and magnesium from the rocks. So this is how this enormous volume of salt was developed over billions of years, making salt water salty ever since.

Fresh remains fresh

But what makes fresh water stay fresh? The water in the lakes and rivers is constantly changing, so there is no time for the salt to accumulate. The opposite is true for oceans and seas; in salt waters, continuous salt replacement is provided by rivers, while water is lost through evaporation only, keeping all of the minerals in them. This is how this tremendous amount of salt could accumulate over the past few billion years.


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