Steve Marsden’s

Most molecules, however, contain more than one covalent bond. So with the possibility of bond moments pointing in a variety of directions, it may not be so clear when there is a net or molecular dipole. That's where the 3-D geometries we have been looking at come in handy.

Although it is possible to determine by graphical means the ultimate direction of a molecular dipole and even to quantify that dipole, our purpose here is simply to be able to distinguish polar molecules from non-polar molecules. For that, all we need is a mental picture of a molecules structure and some basic grasp of symmetry considerations.

Polarity in molecules is responsible for a variety of mundane properties. One of the more useful ones is the solubility of polar liquids and solids in each other. Chemists often say "like dissolves like". This means that polar substances tend to dissolve in polar substances because their molecules can interact strongly. Non-polar molecules will not be able to merge into the "structure" of a polar substance very well and will remain separate, much like hexane and water.