Re: how does salt get in salt water?

This question is probably as ancient, and has been asked as many times, as "why is the sky blue?" Fortunately the answer to the "sea salt question" is probably a bit easier to understand.

When rain or subsurface water comes in contact with rocks, the water dissolves a little bit of the minerals in the rock -- a process called chemical weathering. Therefore, because of chemical weathering, neither surface nor subsurface water is pure, instead they contain lots of dissolved minerals. When this water evaporates the salt gets left behind.

Now consider the oceans for a moment. Every day millions of gallons of rainwater flow into the sea. Meanwhile, millions of gallons of pure water evaporate from the ocean surface, leaving lots of salt behind. So the ocean can't help but be salty.

If more water is removed from the oceans through evaporation than is replaced by river water flowing in, and rain falling over, the oceans, salt layers begin to accumulate on the ocean bottom.

Just imagine for a moment that you've mixed up a batch of three or four teaspoons of table salt with a cup of tapwater. If you let this begin to evaporate it will become increasingly salty over a span of a few days. If you let the water evaporate long enough, salt crystals will form on the bottom and sides of the glass. In fact, if you evaporate enough tap water you'll discover that you don't even need to add salt to form salt crystals. There is enough salt in a few cups of tap water to form a thin layer of crystals.

Of course, most rocks aren't made up of table salt, so ocean water isn't just plain old, garden variety salt water either. Ocean water contains some of every naturally occurring chemical element. So there are lots of different types of salts in ocean water. But the most common type of salt in the ocean is, in fact, sodium chloride -- table salt.

You might wonder why the ocean doesn't just get saltier and saltier over time. Well, the answer is really quite simple, the salts get used in a variety of ways by organisms and in a variety of natural, inorganic processes. For example, clams remove salts from the water to make their shells, and as described above salts form deposits on the ocean floor. Therefore, the oceans maintain a constant salt content of about 3.5%.