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$$\ce{H2O~(l) + NaCl~(s) ->[\Delta] Na+~(aq) + Cl^{-}~(aq)}$$

When table salt is placed in water, it dissolves due to the polarity of water molecules. When solvation takes place, negatively polar sides of water molecules attach to $\ce{Na+}$ ions, and positively polar sides of water molecules attach to $\ce{Cl-}$ ions. The ions are then carried away by water molecules through diffusion. Since the sodium is now isolated from the chlorine, what keeps it from violently exploding with water in the following chemical reaction?

$$\ce{2Na + 2H2O -> 2 NaOH + H2}$$

Also, since chlorine is poisonous, wouldn't that make drinking salted water extremely lethal?

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    $\begingroup$ The first reaction involves Na+ but the second involves Na. They're different species. $\endgroup$
    – gsurfer04
    Jul 29, 2015 at 19:24

4 Answers 4

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[A really-good answer to this question should include quantitative evidence, but here is my hand-waving version...]

Solvated sodium ions ($\ce{Na+}$) aren't all that reactive. Surrounded by a lot of polar water molecules, they're actually quite stable. What you're thinking of is solid sodium (Na) which will "react violently with water" until it's all dissolved - dissolved that is, into stable sodium ions.

Same kind of logic applies to chlorine. Solvated chloride ions ($\ce{Cl-}$) are stable and not nearly as poisonous as chlorine gas. (Note: everything is poisonous at some level, i.e. "the poison is in the dose". In saying chlorine gas is more poison, that really just means it is a more 'potent' poison: it will kill at lower doses compared to chloride ions.)

Another way to think of it: Solid sodium and chlorine gas are at high energy states. Since they can reach much lower energy states by ionzing in the presence with water, the WILL break down and ionize in the presence of water

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    $\begingroup$ Note: chlorine gas actually forms hypochlorous acid with water, but the principle is sound. $\endgroup$
    – Aesin
    Sep 11, 2013 at 21:46
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The answer is that the universe is lazy. And that means that the universe is always seeking the lowest energy states it can find.

And so $\ce{Na}$ is less lazy than $\ce{Na+}$, and similarly for $\ce{Cl}$ and $\ce{Cl-}$, because both these transitions release energy and thus are lazy. $\ce{Na}$ is dangerous, and $\ce{Cl}$ is poisonous, precisely because they have not achieved their most lazy state and thus want to react more, $\ce{Na}$ donating an electron and $\ce{Cl}$ accepting one, both releasing energy.

The "universe is lazy" hypothesis is further supported by the 'most reactions are random" hypothesis. Most molecules are randomly hitting each other, and those that release energy are going to be favored.

And finally, $\ce{Na+}$ and $\ce{Cl-}$ are not dangerous because they are in their lazy states and thus don't need to react with much of anything.

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Sodium metal reacts violently with water because an electron is transferred from sodium to water (or protons in water, depending on pH) to form H2 gas. However, Na+ has already lost its electron, and Na+ does not transfer an additional electron to water.

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Sodium ion Na+ should not be confused with metallic sodium Na. They are actually two different chemical species (atoms vs ions), so it is expected that they chemically behave in a different way. The same applies to chlorine Cl and chloride ion Cl-.

Na can "switch" to Na+ when it is oxidated; this process of oxidation requires a first ionization energy: Na is supplied with energy in order to become Na+, so it is an endoergonic process; a strongly energetic one. When NaCl dissolves in water, the partially negative side of water molecule bonds weakly with Na+ (intermolecular bond, which has electrostatic character); in a qualitative view, forming a bond is an exoergonic process; since this type of bond is weak, the energy lost by Na+ isn't enough to bring Na+ back to Na.

Cl- has a lower energy than Cl has, so when Cl- forms a weak bond with the partially positive side of water molecule, its energy lowers and it's thermodinamically impossible for it to become Cl.

What you asked about poisoning power of Cl is a Biochemistry topic, but it's reasonable thinking that in a healty person's body at room conditions (e.g. not a scuba diver underwater) the total amount of Cl (Cl• radicals or Cl2 molecules) isn't large enough to cause health issues and it won't appreciably raise by drinking salted water. On the other hand, Cl- is part of our daily consumption of mineral salts.

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