Question

Rain water becomes saline

Answer 1

The answer is that the Sun puts energy onto the Earth's surface and that includes the ocean surface, and every square metre gets energy at the rate of, on average, about 1 kilowatt, so 1,000 joules per second. This gives energy to the particles of water in the sea and dissolved in the sea are obviously some ions, sodium and chloride, and lots of others as well, that's why the sea is salty. But the water molecules, although they are sticky and they stick to other water molecules because they are what's called dipole molecules, when you give enough energy to the water molecule, it can break the bonds to the other water molecules, holding it into the water, and it can escape as water vapour.

It isn't possible to give energy in the same way to the ions, or at least not sufficient energy to make them boil off and get into a vapour state like the water. And this is because they are charged and that charge on them makes them a lot stickier, and they interact with other molecules of water, and other ions in the water, far more strongly than a water molecule does. So, it's far easier for water to escape at the temperatures that we see than those other ion species.

They then go up into the air as water vapour until they fall in temperature and this enables them when they also reach a certain height in the atmosphere and interact with other water molecules, dust particles and even dandruff. And they form little droplets, and then you have a cloud. And if you make that air get even colder because for instance, the air has to rise and the air expands as it rises, and the temperature drops, then you get precipitation. The water comes out of the cloud and because just fresh water went up into the air, there's only fresh water to come back out of the cloud, and as a consequence, you get fresh rain. And that's why rivers and streams contain fresh water, but the sea is salty because as the fresh water filters down through the land, it takes a small concentration of salts out of the land and out of the air too, back into the sea where they slowly concentrate.

They're now at sort of steady state concentrations because if the sea were to become any more concentrated, other chemical reactions would kick in and remove some of the salt, or some of the other things as minerals in a solid form. So the sea is saltier now to steady state level of concentration - lovely question.