Dispersion forces (transient polarization of electron clouds) exist between ALL molecules, but it is not the dominant intermolecular force in this instance.
The normal boiling point of #HF# is #19.5# #""^@C#; that of #OH_2# is #100# #""^@C#. These values are absurdly HIGH for such small molecules, and points to a special type of intermolecular interaction, that of hydrogen bonding, which occurs when hydrogen is bound to a strongly electronegative element, such as fluorine or oxygen. Hydrogen bonding thus decreases the volatility of each material.
The heteroatom strongly polarizes electron density towards itself to give a dipole that we could represent as #""^(-delta)OH_2^(delta+)# or #""^(-delta)F-H^(delta+)#. In the condensed phase the dipoles line up appropriately, and this phenomenon is described as #"hydrogen bonding."# Ammonia, #NH_3#, also exhibits some degree of hydrogen bonding. It is the dominant intermolecular force.
Now compare the boiling points of #HF# and #H_2O# with the lower Group hydrides, #H_2S#, and #HCl#, or #PH_3#, where the dipole-dipole is not so strong due to the reduced electronegativity of the heteroatom. What do you find, and how does you account for it?
