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Sodium and chlorine presumably react together strongly, which is why the solid product is stable and takes a lot of energy to break the bonds formed, and to melt. According to the Wikipedia, melting point is about $\pu{800 ^\circ C}$.

So why is $\ce{NaCl}$ so very soluble in water, and when it does dissolve and precipitate, very little heat appears to be involved?

(please note if this seems a simple question, I'm a programmer not a chemist).

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  • $\begingroup$ @Poutnik: re. your 1st coment, I'm not a chemist, I'm a programmer (<stackoverflow.com/users/3779002/user3779002>). I'm afraid my chem questions are going to be n00bish. re. the 2nd, I'd expect what you say would mean dissolving of NaCl would be possible but very slow. $\endgroup$
    – user3779002
    Apr 6, 2022 at 15:59
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    $\begingroup$ I would also comment in saying that the problem of solubility is not yet completely understood. Nobody is able to calculate the solubility of a given substance. There are some rules working for a vast number of substances, but with always plenty of exceptions. You may use thermodynamics parameters, or electronegativities, or hydration degree, or whatever parameter you like, you will never be able to calculate a solubility. For example, why are $\ce{CaCl2}$ and $\ce{NaClO4}$ soluble in nearly their own mass of water, and why $\ce{CaF2}$ totally insoluble and $\ce{KClO4}$ nearly insoluble ? ? $\endgroup$
    – Maurice
    Apr 6, 2022 at 20:04
  • $\begingroup$ As a generality, some chemicals evolve heat when dissolved, e.g., NaOH in water; others absorb heat, e.g., NH4NO3 in water. NaCl happens to be somewhere between those extremes. $\endgroup$
    – DrMoishe Pippik
    Apr 6, 2022 at 23:31
  • $\begingroup$ @Poutnik: If you answer using your "There is balancing of energy.." or similar, I guess that's the closest we'll get to something I understand, so I'll accept that, thanks. $\endgroup$
    – user3779002
    Apr 8, 2022 at 9:59
  • $\begingroup$ My relevant comments were converted with slight change into the answer and deleted. $\endgroup$
    – Poutnik
    Apr 8, 2022 at 10:21

1 Answer 1

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During dissolution of ionic compounds, the thermal outcome is given by balancing of the energy needed to break the ionic lattice and the energy released by the ion hydration, wrapping them by water molecule dipoles.

Nice example of this is exothermic (heat releaseing) dissolution of CaCl2 (where significant energy is released by $\ce{Ca^2+}$ hydration) versus endothermic (heat absorbing) dissolution of $\ce{CaCl2 . 6 H2O}$ where $\ce{Ca^2+}$ is "pre-hydrated". This energy balance is about neutral for $\ce{NaCl}$.

See also Wikipedia: Gibbs free energy as thermodynamic parameter for evaluation of spontaneity of processes, involving both energy outcome and entropy change (in layman terms the level of disorder). Even energy consuming dissolution may happen, if the total entropy (disorder) change of the universe increases by that.

Note that thermodynamics ( energy and entropy changes ) and kinetics (rates of changes) are in large extent independent. Like 2 paths across mountains, one with bigger altitude change but through a lower pass and the other with smaller altitude change, but via a higher pass.

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