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I was reading the 2 following posts:

Altough they both contain wonderful answers, I did not understand if melting can be caused just by the break of the intermolecular weak bonds, while we're heating the substance.

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    $\begingroup$ I have a sugar crystal and smash it into grains. Obviously I broke intermolecular bonds. Did the sugar melt? $\endgroup$ – MaxW Jan 8 '18 at 16:41
  • $\begingroup$ @MaxW Thank you for your insight. I was assuming the substance to be heated up. I edited to make it clearer. $\endgroup$ – Gabriele Scarlatti Jan 8 '18 at 16:46
  • $\begingroup$ Also important to ask is when you break the bonds, do they reform? $\endgroup$ – Zhe Jan 8 '18 at 16:58
  • $\begingroup$ This question is a bit unclear. Could you please elaborate? $\endgroup$ – Linear Christmas Jan 8 '18 at 20:54
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I'm somewhat befuddled as to the confusion. I think it is perhaps the problem is in the phrase "intermolecular weak bonds."

If you look at a diamond crystal there aren't any diamond "molecules." Rather the whole crystal is one big "molecule."

Likewise we write the chemical formula for sodium chloride as $\ce{NaCl}$, but in solid and liquid sodium chloride there are no sodium chloride molecules. True sodium chloride molecules can only exist in the gaseous state.

If you look at $\ce{H2O}$ the hydrogen atoms are extremely labile. So the two hydrogen atoms and the one oxygen atom that were a molecule when the ice froze might not be part of the same molecule when the ice melts.

So to fit the overall criteria of "intermolecular weak bonds," we need a molecule which has stronger intramolecular bonds than intermolecular bonds. Say methane for example. So when methane freezes the same molecule that freezes in the solid will be the same molecule that is freed when the solid methane melts.

So in the case of methane adding energy in the form of of heat to solid methane will eventually reach the melting point of methane. At that point the weaker intermolecular bonds of the crystal break and the bulk solid turns into liquid methane.

So the overall notion is that the solid form exists as colder temperatures than the liquid form and that the liquid form exists at cold temperatures that the gaseous form. So generally we take solid and heat it. When (IF!) it turns to a liquid then the temperature of the transition is the melting point. Likewise if the liquid can be turned into a vapor then the temperature of the transition is the boiling point. Thus solid methane and sodium chloride will both "melt," but for different reasons.

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  • $\begingroup$ Yeah It was the "intermolecular weak bonds" phrase that created confusion, I did mean just intermolecular bonds. Btw thank you very much for your answer, I did not understand if you consider Diamond to have intermolecular bonds. And I'm little bit confused, because your answer seems to suggest that not every susbstance melt by breaking "intermolecular bonds", but the answer below seems to express a different view, in some way. I would love some clarification. $\endgroup$ – Gabriele Scarlatti Jan 9 '18 at 9:45
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    $\begingroup$ @GabrieleScarlatti - The point is that a diamond crystal doesn't have discrete molecules in its crystal structure. So with no discrete molecules there are not any intermolecular bonds which need breaking. Same thing for NaCl. $\endgroup$ – MaxW Jan 9 '18 at 10:22
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When you heat a substance to the melting point, all permanent intermolecular (or inter-ionic) bonds break. That's why you call it melting point. You cannot just randomly break bonds, except by heating to the melting point, so it's pointless to ask what happens if you break intermolecular bonds.

Of course you can break bonds at a lower temperature (by radiation, aggressive chemicals, etc), but then you destroy your sample, not melt it.

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  • $\begingroup$ Thank you for your interesting answer, btw where did I ask "What happens if we break intermolecular bonds?". $\endgroup$ – Gabriele Scarlatti Jan 9 '18 at 9:47

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