In the liquid and solid states, molecules are held together by attractions called intermolecular forces. there are several types of intermolecular forces. london dispersion forces, found in all substances, result from the motion of electrons. these work to attract both polar and nonpolar molecules to one another via instantaneous dipole moments. dipole-dipole forces arise from molecular dipole moments. ion-dipole forces result from the interaction of an ion and a molecular dipole. hydrogen-bond forces result from the attraction of a hydrogen atom bonded to a small highly electronegative atom (n, o, and f) and the unshared electron pairs of another electronegative atom physical properties such as boiling point, melting point, vapor pressure, viscosity, and surface tension are all affected by the strength of the intermolecular forces within a substance. part a what happens to these physical properties as the strength of intermolecular forces increases?