Why the sea is salty

Rain that falls on land is nearly fresh. As it runs over rock and soil it dissolves small amounts of minerals, including salts. Rivers gather that water and deliver it to the sea.

At the surface of the ocean, water evaporates into the air and later falls again as rain. Evaporation takes the water and leaves the dissolved salts behind. Over a very long time, the ocean has become salty.

What rivers deliver

The main salt is sodium chloride, the same chemical as table salt, though seawater also holds magnesium, calcium, sulphate, and many other dissolved substances. The exact mix is similar in oceans around the world because currents stir them.

Not every body of water works this way. A lake with an outlet keeps flushing salts downstream, so it stays fresh. The Dead Sea and other salt lakes have no outlet. Water only leaves by evaporation, so the salts concentrate.

Why lakes are usually fresh

Sea ice is mostly frozen water, not frozen brine. When seawater freezes, much of the salt is left in the remaining liquid, which becomes denser and sinks. That sinking helps drive deep ocean currents.

You cannot drink seawater to quench thirst. The salt content is high enough that your body would use more water getting rid of it than the drink provides. Fresh water is the useful supply for people and for most land animals.

What rivers deliver

Rivers carry dissolved minerals to the ocean. Water leaves by evaporation. The salt stays. Rain that falls on land is nearly fresh. As it runs over rock and soil it dissolves small amounts of minerals, including salts.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. Why the sea is salty needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

Why lakes are usually fresh

As it runs over rock and soil it dissolves small amounts of minerals, including salts. Rivers gather that water and deliver it to the sea.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

Rivers gather that water and deliver it to the sea. At the surface of the ocean, water evaporates into the air and later falls again as rain.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

At the surface of the ocean, water evaporates into the air and later falls again as rain. Evaporation takes the water and leaves the dissolved salts behind.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

Evaporation takes the water and leaves the dissolved salts behind. Over a very long time, the ocean has become salty.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

Over a very long time, the ocean has become salty. The main salt is sodium chloride, the same chemical as table salt, though seawater also holds magnesium, calcium, sulphate, and many other dissolved substances.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.

Condition, then outcome

The main salt is sodium chloride, the same chemical as table salt, though seawater also holds magnesium, calcium, sulphate, and many other dissolved substances. The exact mix is similar in oceans around the world because currents stir them.

The first statement can be true and still not finish the job. The second statement is the job finishing. Hold both, and the paragraph above stops being a list. It becomes the reason the title is worded as it is.

What the title is naming

The exact mix is similar in oceans around the world because currents stir them. Not every body of water works this way.

If you wanted a shorter version, it would be those two sentences and nothing else. Everything added around them is there so the dependence is obvious: the second does not happen unless the first has happened.

What has to come first

Not every body of water works this way. A lake with an outlet keeps flushing salts downstream, so it stays fresh.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. Why the sea is salty needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.