Question

What causes dipole-dipole forces?

Answer 1

The simple answer is molecular dipoles.

Explanation:

Dipoles result from unequal sharing of electronic density in a molecule. And so typically they are observed for molecules in which there are one or more strongly electronegative atoms. And by way of example compare the volatilities of chloromethane, a polar molecule, normal boiling point `-24.2` `""^@C`, with non-polar methane, normal boiling point `-164.0` `""^@C`. As always the dipole moment is the vector sum of the individual bond dipoles.

In the bulk phase, the dipoles interact to constitute an intermolecular force, that should be expressed in the individual normal boiling points of each molecule.