Dipole-dipole interactions

Dipole-dipole interactions exist between molecules that are polar.   This requires the presence of polar bonds and a unsymmetric charge distribution in the molecule.   These molecules have a permanent separation of positive and negative charge. In the illustration the H end of HCl is permanently slightly positive charge. The Cl end of HCl has a permanent slight negative charge. the "H" in one molecule is attracted to the "Cl" in a neighbor. The intermolecular force is weak compared to a covalent bond. But this dipole-dipole interaction is one of the stronger intermolecular attractions.