Why is some water on Earth salty, and some fresh?

What causes the divide?
27 October 2023


A breaking wave on the ocean


Thanks to listener Joanne for the question.

As anyone who’s ever taken a dip in the sea and had the unfortunate experience of swallowing some water will know, it tastes incredibly strongly of salt.

And you’re absolutely right, this is because of minerals from rocks on land making it into the sea.

There are a few ways this happens: one of the main ones is that carbon dioxide in the air dissolves into rainwater, making it slightly acidic. When this rain falls, physical and chemical processes release minerals including sodium and chlorine from the rocks into the runoff water which either goes directly into the sea, or is eventually transported there via rivers, streams and lakes.

So rivers, although they might not taste of salt, do have these minerals in them, but they’re being carried in a continuous flow through the water system towards the sea by gravity, meaning the saltiness never accumulates.

Similarly, most lakes also have this avenue for escape for the water and minerals via the rivers and streams connected to them, so the salt doesn’t have a chance to collect there in vast quantities either.

So all roads lead to the sea, where, as Paul writes under the forum post for this week’s Question of the Week: ‘Water evaporates leaving whatever salts are present in the oceans and the evaporated water then becomes precipitation to start the cycle over again.’ And he’s exactly right. Thanks for that, Paul.

The main way water escapes is through the chemical process of evaporation. The difference with this escape route is that there’s no room for the minerals dissolved in the water to make the jump too. As water molecules with enough energy escape from the surface of the ocean into the air above, left behind sodium and chlorine raises the salt concentration of the sea.


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