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)?