Uncharged molecules may either be polar or non-polar depending on both on their geometry as well as the electro-negativity of the constituent atoms. If the centre of total positive and negative charges in a molecule do not coincide (for example, for water), it results in a permanent dipole, which imparts a polarity to the molecule. Conversely, molecules for which the centres of positive and negative charge coincide (for example, methane) do not possess a permanent dipole and are termed non-polar. However, even a so-called non-polar molecule, may possess an instantaneous dipole for the following reason. At the atomic level as electrons oscillate about the positively charged central nucleus, at any point of time a dipole is set up. However, averaged over time, the net dipole moment is zero.

When two polar molecules approach each other closely the electric fields of the dipoles overlap, resulting in their re-orientation in space such that there is a net attractive force between them. If on the other hand a polar molecule approaches a non-polar molecule, the former induces a dipole in the latter (due to displacement of the electrons from their normal position) resulting once again in a net attractive interaction between them. Lastly when two non-polar molecules are close enough their instantaneous dipoles interact resulting in an attractive force. Due to these three types of interactions (dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole) molecules of any substance or a mixture are subjected to an attractive force as they approach each other to very short separation distances.