Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up.
Sign up to join this community
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.
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.
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.
