Two kinds of forces exist inside solids. Inside each molecule the strong -molecular forces are bonds. Between the molecules are much weaker -molecular forces. It is these between-molecule forces which are when the solid melts or the liquid boils.

Inter-molecular forces are divided into two types: hydrogen bonds, and van der Waals' forces.

Hydrogen bonds exist in compounds containing a atom bonded to an O, N or F atom. The presence of hydrogen bonds in a solid indicates relatively melting and boiling points.

Van der Waals' forces occur within all substances due to the presence of temporary or permanent within the molecule.

The within each molecule are in constant, random motion. Sometimes there will be a greater of electrons in one part of the molecule, with fewer in another part. That creates regions of negative and positive , which in turn induces a region of negative and positive charge on a nearby . A slight attraction will then exist between these two molecules.

The greater the number of , and the greater the area of the molecule, the more opportunities there are for temporary to occur. Thus, larger molecules have van der Waals' forces between them than small ones, and therefore higher melting and boiling points.

The molecules are able to pack together, the stronger the between them. -chained alkanes therefore have stronger forces between them than their -chain isomers. The C=C bond in alkenes is unable to , so alkene molecules are more rigid and difficult to fit together than alkanes. alkenes have a 'C'-shaped kink in them, making them much more awkward than their equivalents.

Polar compounds, which have a dipole in addition to the temporary dipoles which may form elsewhere within the molecule, have a stronger intermolecular forces than compounds of similar molar and shape. The contribution made by the permanent dipole is relatively , so where shape and mass differ as well as polarity, shape and mass have a effect on melting and boiling point than polarity does.