What is an attraction between molecules of different substances?
Dispersion forces such as Van der Waals forces occur due to momentary dipoles that occur in molecules due to random uneven distribution of electrons. This means that from moment to moment, it is possible to have a greater density of electrons on one side of the molecule than the other (of course this averages out over the longer term). This creates an uneven charge distribution that induces a similar effect in nearby molecules, and the molecules are able to interact, albeit weakly.
Substances that rely on dispersion forces generally are low melting solids or liquids and gases at room temperature.
A hydrogen bond occurs in a molecule in which a hydrogen atom is bonded to an atom that is both small and electronegative (meaning F, O or N for the most part). This causes the bond between these atoms to be very polar, and the H atom to become quite positive.
Then, this H will be attracted quite strongly to another molecule that has a similar F, O or N atom (due to the resulting partial negative charge on this atom). The hydrogen bond is typically one-tenth the strength of a covalent bond. It is the effect that causes the bases in a DNA molecule to link together (as one example).
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