Science Questions

Why isn't rain salty?

Sat, 22nd Sep 2012

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Question

Eddie asked:

Hi Chris

 

Just want to know that if rain water originates from the sea and sea water is salty, why than don't the rain taste salty, what happens to the salt content in the process?

 

Your assistance would be highly appreciated.

 

Regards

 

Eddie

Answer

RainChris -   The answer is that the Sun puts energy onto the Earthís 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, thatís 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 isnít 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, itís 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 thatís 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 Ė lovely question.

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The boiling point of Sodium Chloride is about 1413 įC.  Likewise other minerals dissolved in seawater has relatively high boiling points. 

So, the partial pressure exerted by the salt dissolved in the seawater is far less than that of the water.  And, thus, while the water evaporates, the sodium chloride (salt) does not, and remains behind in solution.

Some salt does get swept up by the wind and wave action of the oceans.  But, that salt tends to fall within a few miles of the coastline, and once fallen, it doesn't re-evaporate to fall as salt water elsewhere.  But, this windblown salt does case significant corrosion to metals along the coastlines. CliffordK, Tue, 18th Sep 2012

And the salt story is even a bit more interesting than that. Some salt gets swept up not in spray from the waves, but in tiny droplets of sea water that form when the bubbles in waves burst at the surface. The water evaporates or not forming a colloidal aerosol that does not easily settle out. Whether a solid or a liquid aerosol depends on magnesium content -- high magnesium = liquid, low Mg = solid. These particles, known as "halites" are carried on air currents for several days. Some are dry deposited, and can lead to salinity in desert surface soils over a few million years. But the more important role of halites is in helping to form clouds.

In the atmosphere, all of the little molecules of water vapour tend to move around individually. They do not often come together and stick together. A water droplet or ice crystal cannot really start to form from pure water vapour unless 50 to 100 consenting water molecules meet together at the same time. But a halite can attract water molecules onto its surface one at a time and gradually build up the sorts of water or ice droplets that are in clouds. The process is called "nucleation".

Rain water is very very slightly salty. The purest water you can make contains about 10-15 mole, or 1 billion atoms of sodium per litre (and only slightly smaller amounts of other common ions, magnesium, chloride, and sulfate, and even larger amounts of bicarbonate). damocles, Fri, 21st Sep 2012

An experiment that you can do to model this phenomenon, and there is a good chance that you have done this before, is to take a pot of water and bring it almost to boiling.  Then take it off the stove/heat source and pour it into another heat safe container, if you desire (for ease of cleanup, it's better if you can throw it away). Now start to slowly pour salt in while stirring, pouring as much in as possible but stopping before the solution becomes over-saturated (when grains of salt begin to collect on the bottom and won't dissolve).  Set this solution preferably in a warm, sunny for a few days until all the water evaporates, though a cooler, more shaded place will work, as well.  Once all the water has evaporated, a crystal matrix of salt crystals may have formed or simply a white crust of salt crystals (which, unfortunately, is probably more likely).  If you don't feel like waiting, you could always make the solution in the pot and keep it over the stove/heat source until it boils down.  I don't really recommend this, though, for a few reasons...
-The salt might take a while to dissolve again, unless you scrub it, which would still probably be a little difficult (and could damage a non-stick pot)
-If it is a non-stick pot:
  -The finish on it could be damaged
  -Toxic fumes can be produced from the non-stick coating of pots if they are heated without liquid in them
Lab Rat, Fri, 30th Nov 2012



You could also take a bit of salt and bind it to a fishing wire, then place that hovering in a solution of saltwater, and just let it evaporate, the salt will make a large crystal on the already hovering salt crystal (:

Good ol' memory's from high school (: SorryDnoodle, Thu, 6th Dec 2012

if you want to prove that the salt does NOT evaporate with the water, then you should use the fishing wire procedure in the last posting, but suspend salt from 2 wires -- one dipping into the saturated solution, and the other held just above it but not touching the water. Salt crystals should grow on the first but not the second. damocles, Mon, 10th Dec 2012

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