Final answer:
HNO₃, or nitric acid, has the largest dipole-dipole forces due to its polar nature and the ability to form hydrogen bonds, compared to the other given substances.
Step-by-step explanation:
Among the substances Ar, Cl₂, CCl₄, and HNO₃, the one that has the largest dipole-dipole forces is HNO₃ (nitric acid). While Ar (argon) is a noble gas with only London dispersion forces, Cl₂ (chlorine gas) and CCl₄ (carbon tetrachloride) are nonpolar molecules, also primarily experiencing London dispersion forces. HNO₃, however, is a polar molecule due to the presence of a significant difference in electronegativity between the nitrogen and the oxygen atoms, and the asymmetrical shape of the molecule, which leads to a net dipole moment. This causes HNO₃ to experience strong dipole-dipole interactions. Moreover, HNO₃ is capable of forming hydrogen bonds, which is a specific and strong type of dipole-dipole interaction.