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From just the IB chemistry bonding chapter information, you would normally deduce that $\ce{PCl3}$ and $\ce{PCl5}$ are both covalent molecules.

However, as $\ce{PCl3}$ has a dipole moment, it has dipole-dipole intermolecular attractions, implying that $\ce{PCl3}$ has a higher melting point than $\ce{PCl5}$, which only has London dispersion forces.

So why is it that $\ce{PCl5}$ has a higher melting point than $\ce{PCl3}$ (I saw online that $\ce{PCl5}$ is an ionic solid)?

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Because $\ce{PCl5}$ does something which is not immediately obvious from its molecular formula: it autoionizes and becomes an ionic solid $\ce{PCl4+PCl6-}$. As such, it has much stronger interactions than $\ce{PCl3}$ with its mere dipole-dipole attractions, hence the higher melting point.

If not for that fact, you deduction should have worked just fine.

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  • $\begingroup$ Is there any mathematical way or just method to deduce that $\ce {PCl5}$ is an ionic solid? Or is that from observation. $\endgroup$ – Stone Dec 12 '17 at 15:23
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    $\begingroup$ No, I don't think this can be deduced via logic alone. As for more mathematical way, in chemistry this usually implies that you are going to end up in a nocturnal abyss of quantum chemistry, which is not what you want, believe me. $\endgroup$ – Ivan Neretin Dec 12 '17 at 15:33

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