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Why doesn't chlorine in seawater evaporate?

  1. Mar 24, 2013 #1
    I'm having trouble to understand this.

    Let's see, as far as I've read, salt in seawater is not present as NaCl molecules but in the form of dissolved (solvated) ions Na+ and Cl-.

    Provided that chlorine is a gas at ambient temperature, why don't these Cl- ions just evaporate (alone or together with H2O) out of the sea?

    I'm sure I'm getting something deep wrong, but I'm still curious. :smile:

    Thank you all!
  2. jcsd
  3. Mar 24, 2013 #2
    Correct me if i'm wrong but surely its because the ions are part of the ionic compound so are held together by the strong electrostatic attraction (two opposite charges attract).
    So to be released from this the ions have to become atoms, this is the whole point of electrolysis as the anode and cathode give/take electrons to return the ions back to atoms which are then released.
    This link may help www.science.uwaterloo.ca/~cchieh/cact/c123/eltlysis.html
  4. Mar 24, 2013 #3
    Or you could think of it in the context of salt farms. The water evaporates of leaving the salt behind which is collected. So the salt cannot evaporate of with the water (probably because its too heavy)
  5. Mar 24, 2013 #4
    Actually, this is another thing I don't exactly understand. Salt is formed when water evaporates and the ions 'meet' or is there another mechanism there?
  6. Mar 24, 2013 #5
    I think salt was already formed in the water just dissolved, are you confused about why atoms for ionic bonds?
  7. Mar 24, 2013 #6
    Might help to post this in chemistry section though
  8. Mar 24, 2013 #7


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    Staff: Mentor

    Cl- is a chloride, not chlorine. Same element, but different properties.

    Yes. the idea that ions "meet" is a reasonably correct one. This is just opposite to dissolution - when you put a salt crystal into water, it dissolves and dissociates, when you evaporate the water ions "meet" to rebuild the crystal lattice.
  9. Mar 24, 2013 #8
    Seems I'm utterly confused on how ions work. :shy:

    To my understanding (half-forgotten from high school and most probably wrong) ions are 'free atoms' with a charge.

    So my reasoning is, 'hey, here we have all these free chlorine atoms in seawater... huh, since chlorine boiling point is -34ÂșC, why don't they just evaporate out of the water?'

    Please correct me in any stupid thing I'm saying! :wink:
  10. Mar 24, 2013 #9
    I think your taking free wrong. Free means free to move within the solution. They use free for electrolysis as an ionic compound can only conduct electricity when a liquid and when the ions are free
  11. Mar 24, 2013 #10
    Thank you so much, Borek. :smile: Just out of curiosity, where can I find these physical properties of chlorides like melting or boiling points?

    Got it. I'm still trying to get why saltwater tastes salty if there are not salt molecules but dissolved ions in it. :biggrin:
  12. Mar 24, 2013 #11
    Oh I wouldn't try to think about that as i don't think taste and smell is not properly understood
  13. Mar 24, 2013 #12


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    Staff: Mentor

    You can't. You can find properties of salts - like sodium chloride, lithium bromide and so on. There is no such thing as separate chloride, they are always accompanied by a counterion. Salt (or the solution) on the whole is electrically neutral.

    Imagine a huge dance, in a closed hall, with a thousand participating pairs. People are all in the same ballroom, and they can freely move - but each has its counterpartner (doesn't have to be the same all the time, but there is always 1:1 ratio between guys and guyettes) and they are confined to the room (ie, you can't remove guys only). That's more or less how the solution works.

    Actually it is the other way around. The taste is that of dissolved and dissociated salt. When you lick the crystal it dissolves in the saliva, and you are reacting to the presence of ions.
  14. Mar 24, 2013 #13
    Thank you again. :smile:

    I see. I have read that nuclear tests in the sea produced huge amounts of (I supposed gaseous) Chlorine-36. I assume that the instant vaporization of saltwater at ultra-high temperatures could keep those ions 'free' somehow or forming Cl2 or the like? Or maybe it could be just 'neutron-activated nuke-propelled-into-the-atmosphere' plain salt? :wink:
  15. Mar 24, 2013 #14


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    Staff: Mentor

    Isotopes produced in the nuclear reactions are usually highly ionized, so they are impossible to classify as "chloride" or "chloride" or something else. But once everything settles down (read: cools down) chloride anions are the most likely (and most stable) form these atoms can take.
  16. Mar 24, 2013 #15
    I think the simple answer is electrostatic attraction. When a Cl- ion gets enough kinetic energy to get kicked off the surface of the water it leaves the solution with a net + change, so it gets pulled back in.
  17. Mar 24, 2013 #16
    Opposite charges attract, similar charge repel.

    Chlorine ions are not like chlorine molecules. Chlorine ions are negatively charged. Chorine molecules are neutral.

    Salt dissolved in water is dissociated into sodium ions (Na+) which are positive and chlorine ions (Cl-) where are negative. Like charges attract.

    The sodium ions attract the chlorine ions because they are oppositely charged. The only reason they don't stick in water is that the water molecules sequester the ions. If a chlorine ion were to leave the water, the sodium ions would pull it back. Similarly, a sodium ion that leaves the water will be pulled back by the chlorine ion.

    Chlorine molecules are neutral. When one leaves the water, there is nothing to pull it back.
  18. Nov 2, 2015 #17
    Darwin123 has it just about right. Chlorine, the element, is a gas, formed of molecules of Cl2. It is highly reactive, because neutral chlorine is much happier when it grabs an electron from the nearest available "reducing agent" (dark colored stains on your clothes, for example ... Clorox in action!). It's so reactive, in fact, that there is no free Cl2 in nature. You find it only with the extra electron, as chloride ions. To remove that electron, and oxidize chloride to chlorine, takes a lot of energy. Humans are clever enough to do it in "chlor-alkali" plants, using electricity to rip away those electrons, but it doesn't happen in nature. At least, not in seawater, where chloride remains as non-volatile chloride, and never becomes volatile chlorine.
    (It does happen, amazingly enough, in human neutrophils, which essentially make their own bleach to kill their targets.)
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