Which type of forces occurs only between polar molecules?

Dipole-dipole forces are interactions that occur specifically between polar molecules, which have regions of partial positive and negative charges due to unequal sharing of electrons within their bonds. These forces arise because the positive end of one polar molecule is attracted to the negative end of another polar molecule.

In a polar molecule, the unequal distribution of electron density between different atoms leads to a permanent dipole moment, which is crucial for the existence of dipole-dipole interactions. These forces can significantly influence the physical properties of substances, such as boiling points and solubility in polar solvents.

Other types of intermolecular forces, such as dispersion forces, ion-dipole forces, and hydrogen bonding, involve either nonpolar molecules or interactions with ions. Dispersion forces are present in all molecules but are particularly significant in nonpolar molecules; ion-dipole forces occur between charged ions and polar molecules; and hydrogen bonding, while a strong type of dipole-dipole interaction, requires specific conditions involving hydrogen atoms bonded to highly electronegative elements. Therefore, dipole-dipole forces are unique to interactions between polar molecules.