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If you crack a large crystal of a salt like NaCl, why don't the pieces spontaneously bond back together on contact? The bonding is an exothermic process and the charged ions are presumably stuck approximately right where they were before the facture.

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    $\begingroup$ Have you considered how accurately you should put together the fragments so the charged ions facing to the right ones again? $\endgroup$ – Greg Dec 7 '16 at 3:58
  • $\begingroup$ No, but I figured that you certainly are going to be able to get positive ions and negative ions in the same vicinity. And the same would work for metals. I mean I know id doesn't work, but wondering if there is a precise explanation. Does it take some energy to get them arranged to bond with each other. It is connected with my question about whether a pure synthesis reaction can be endothermic. $\endgroup$ – Joseph Hirsch Dec 7 '16 at 4:18
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    $\begingroup$ I have to read this one chemistry.stackexchange.com/questions/5915/… to see if it mine is a dup. $\endgroup$ – Joseph Hirsch Dec 7 '16 at 4:19
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You would have to get the two surfaces (or rather their crystallographic axes) together at the atomically perfect original angle to each other. Otherwise no overall (ionic) binding energy will result: Even if the surface is perfectly even, half the ions will be at places where they feel repulsion rather than attraction by the other side.

With polished metal surfaces, this works much better, also because metal atoms are (in "average") more mobile on a surface than ions.

Another point is humidity. Many surfaces, and certainly ionic substances, adsorb a layer of water from the air, practically instantly. It'll take quite a while before that has diffused out again when you press the surfaces against each other.

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  • $\begingroup$ Not only humidity but also oxygen and nitrogen. Every solid surface has a layer of air gases adsorbed on it. $\endgroup$ – vapid Dec 7 '16 at 8:19
  • $\begingroup$ @vapid No they don't. I've done the BET experiment numerous times, it needs significant cooling (LN2). $\endgroup$ – Karl Dec 7 '16 at 23:21
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    $\begingroup$ Well yes, it depends on temperature and pressure. To get a layer of adsorbed gas you typically need very low temperatures, especially for smooth surfaces. But even at normal pressure and temperature gases are adsorbed on isolated spots (cracks, pores, etc.) on a surface. Try heating an MS flight tube soon after venting (when pressure is still a bit high) and you will see a nice surge of nitrogen signals. $\endgroup$ – vapid Dec 8 '16 at 8:14
  • $\begingroup$ Ah. You said "layer" above. ;-) $\endgroup$ – Karl Dec 8 '16 at 23:00
  • $\begingroup$ OK, my mistake;-) $\endgroup$ – vapid Dec 9 '16 at 7:53

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