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If I melt $\ce{NaCl}$, I get a molten salt. This means a liquid formed only by ions ($\ce{Cl^-}$ and $\ce{ Na^+}$). That could mean that the crystal $\ce{NaCl}$ experimented a chemical change: $$\ce{ NaCl->Cl- + Na+}$$ Like a chemical reaction... The ions have now more space to move and interact with much more ions than in the solid phase. I am trying to get an explanaition why the melting temeprature and the melting heat of molten salts show some erratic behaviour, I mean they do not follow a simple pattern such as that found for n-alkanes or n-alkanols.

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In fact the lattice structure of $\ce{NaCl}$ is composed by $\ce{Na^+}$ and $\ce{Cl^-}$ even when is in the solid state we don't write it with the charge only for convention.

Ionic compounds are different from organic compound, they have complex lattice structure and are in fact compose by two species. Breaking a ionic bond required more energy than an organic compound where you deal only with intramolecular forces.

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    $\begingroup$ Your last sentence is a little bit dangerous if taken at face value. In an ionic lattice, the atoms have an "ionic bond" with every other atom in the lattice, no matter how far. It turns out however that most of the electrostatic attraction comes from the counterions in the immediate surroundings That attraction is very strong, however. In a gas of an ionic compound, you will actually often find ion pairs and a few larger agglomerates even at very high temperatures. Only at massive temperatures (>5000 K) should you expect the last ionic bond between ion pairs to give way, producing free ions. $\endgroup$ – Nicolau Saker Neto Feb 18 '14 at 23:41
  • $\begingroup$ @NicolauSakerNeto which sentence? Breaking a ionic bond required more energy than an organic compound where you deal only with intramolecular forces.? Do you mean when you melt salts you don't break ionic bond? Thanks for the comment $\endgroup$ – G M Feb 18 '14 at 23:47
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    $\begingroup$ In a liquid ionic compound, you "break the bond" with the furthest ions, in the sense that the liquid has no long-range order as it does in the solid. But each ion is still quite tightly "solvated" by ionic bonds with nearby counterions. The attraction between two oppositely charged ions is so strong that it survives even when you turn the salt into a gas. $\endgroup$ – Nicolau Saker Neto Feb 19 '14 at 0:08
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    $\begingroup$ In sodium chloride, the ionic bond between a single $Na^+$ and a single $Cl^-$ is approximately half the lattice energy of the entire solid. That means an ionic bond energy of about $400 kJ/mol$, comparable to the strongest covalent single bonds. This arguably means the ionic bond still survives until about when you expect covalently bound molecules to thermally decompose into separate atoms, which requires huge temperatures. $\endgroup$ – Nicolau Saker Neto Feb 19 '14 at 0:10
  • $\begingroup$ @NicolauSakerNeto Thanks for the comment, I should explain better what I mean for two species what I would underline is the fact that the interaction is not between identical compounds (such an organic solid) but between a cation and an anion (of course a series of each other). $\endgroup$ – G M Feb 19 '14 at 0:23

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