Why isn’t the ocean fresh water?
In: Chemistry
Some rocks have salt in them. Water can dissolve salt. You might know that water always goes downhill. When it rains the water runs downhill over rocks and stuff all the way down to the ocean (because the ocean is the lowest spot on the Earth). That’s how the salt gets in there and then it can’t get back out.
Because it’s full of salt. Rain comes from evaporation which seperates out the salt, and natural springs filter the salt out through all that earth.
Sodium, Na, and chloride, Cl are abundant on earth and naturally bind together making sale. The presence of these two elements on earth largely corresponds with the [distribution of elements in the universe](https://en.wikipedia.org/wiki/Chemical_element#Origin_of_the_elements). Also, any salt that the rainwater and such picks up and flows into the ocean largely stays there. It’s kind of a one-way collection. Salt mines are sections of ocean that got trapped and dried out.
Ocean is filled by all the rivers flowing downhill from mountains to plains to ocean. The rivers are called freshwater but they do have a teensy tiny amount of dissolved salt from the rocks and soils. The rivers carry that tiny bit of salt to ocean. That’s what makes the ocean salty
There is salt in it.
When it evaporates to make rain, the salt is left behind. Fresh water in rain mostly flows into the sea, balancing the salt. Some disolves more chemicals, slowly making the oceans saltier.
Because it is continually refilled by rivers, as well as rain.
Rivers erode rocks and carry salts downstream with them. The ocean is the ultimate “downstream”, where everything accumulates. Once the salts are in the water, it is hard to remove them. It is especially hard to remove them when more salt is being added all the time. The oceans are very, very, very old and have had a long time to accumulate salts.
Ponds and lakes are much younger than the oceans and have more rainfall refill compared to the oceans.
For examples of waterbodies getting salty/brackish over time, look up the Aral Sea and the Salton Sea. Great Salt Lake is another interesting example where there is a lot of evaporation, but little rainfall, resulting in a salty inland waterbody.
As others have mentioned, the main input of salts into the ocean is from rivers which run off the continents and carry with them dissolved ions from the minerals in continental rocks.
Another input is the mineral constituents pumped into the deep oceans at hydrothermal vents.
However, no other responses here have yet mentioned the *removal* of salts from the oceans. Salts are removed through sedimentation onto the seafloor (either directly or via dying micro-organisms which make it to the seafloor sediments), or directly into the oceanic crust when seawater is moving through it before it comes out at hydrothermal vents with a completely different mineral profile. That is to say, hydrothermal vent systems both remove and add various different chemical species to and from the oceans.
This is summarised succinctly in an old r/askscience post to a similar question [here](https://reddit.com/r/askscience/comments/3c9u7f/since_evaporating_ocean_water_is_pure_does_this/cstrbso), and a few more details on the subject gone through in a slightly less old r/askscience post [here.](https://reddit.com/r/askscience/comments/7o1hf2/are_oceans_becoming_increasingly_saltier/ds6fiyk)
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