When water evaporates from the ocean, of course the minerals such as salt remain in the ocean-- that's why seawater is so saline. However, I'm very puzzled: why don't minerals evaporate along with the water? Could you give me a somewhat simple answer for this? (I never studied chemistry, so I really have no clues as to the possible answer.)

  • $\begingroup$ The definition of evaporation is "the process whereby atoms or molecules in a liquid state gain sufficient energy to enter the gaseous state." Does that mean minerals can evaporate also if they are imparted enough energy? $\endgroup$ – George A. Solodun Mar 3 '17 at 20:19
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    $\begingroup$ You don't want to breathe deeply over an open container of molten iron... Now, it is somewhat unlikely that a mineral (normally a fairly complex crystal with multiple elements) would go in to the gas state as a single molecule mimicking the mineral composition. $\endgroup$ – Jon Custer Mar 3 '17 at 20:33
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    $\begingroup$ Related: What happens to the molecules of a liquid when it evaporates? $\endgroup$ – user42086 Mar 3 '17 at 22:16

In order to get a measure of how much a compound is prone to evaporation one can check boiling points. Boiling point of water is 100 C in atmospheric pressure, for molecular NaCl, sodium sulfate and Magnesium chloride (main minerals in seawater) boiling point is above 1400 C. For ionic salts dissolved in water there is a different story since they are ionized. The ions are surrounded by water molecules in liquid phase, while if vaporized they would not have a chance of having water molecules closeby so they prefer to stay in the liquid phase. All of that being said, one can envision a certain temperature, pressure and concentration (extremely high for three) at which ions would somehow get out to the vapor phase. Since sea level temperature is quite low, mineral or salt evaporation is practically zero.

Side note on why would water evaporate at 4 C instead of 100 C: Although it take 100 C of kinetic energy (molecule's velocity) to have "all" water molecules to escape from the intermolecular attractive forces, still "some" of them end up having the energy to escape at 4 C (sea temperature average). It is because kinetic energy has distribution over all the molecules, and you will always find some that have the necessary energy. At the same time "some" of the water molecules in the air end up not having enough kinetic energy to remain in the air, and so they end up being attracted back to the liquid. These two numbers (The molecules of water in air that condense back and the water molecules in the ocean that evaporate) are equal so we have thermodynamic equilibrium. The net number of molecules that escape the sea depends on kinetic energy distribution which is exemplified in temperature.

  • $\begingroup$ I think the part prior to the side note makes a good answer. The side note is a bit shaky , i.e. things like "100 C of kinetic energy, etc., and really isn't about what the OP asked. In short, I get the point of the side note, but I think it takes away from the quality of the answer. My 2 cents. $\endgroup$ – airhuff Mar 3 '17 at 23:20
  • $\begingroup$ Very good answer. Could you give me a website where I can find the melting/boiling points of various minerals? This website gives me the states of the elements at any temperature; however, it will not let me search for compound states at various temperatures. I'd really appreciate being directed to a website that will give me this information. $\endgroup$ – George A. Solodun Mar 5 '17 at 3:42
  • $\begingroup$ NaCl dissolved in water is ionized; why does the NaCl molecule have to be the vapor composition? $\endgroup$ – Whit3rd Mar 5 '17 at 6:07
  • $\begingroup$ Whit3rd is right. It is not much relevant to think about NaCl molecule being in the vapor phase in normal conditions. I should amend my answer.The ions are in lower potential when surrounded by close water neighbors. In gas phase they are not surrounded by close water molecules, so they stay in the liquid. I am trying to put up a case for when you see ions in the vapor. It maybe possible at very high temperature and pressures. Maybe at very high temperatures they get back to elemental form or even molecular form when there is less water around. $\endgroup$ – Kinformationist Mar 5 '17 at 17:20

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