How do these forces affect the chemical and physical properties of molecular compounds?

The strength of intermolecular forces present in 
a substance is related to the boiling point and melting point of the substance.  Stronger 
intermolecular forces cause higher melting and boiling points since more kinetic energy is needed to break the bonds.   
EXAMPLES:   
CH4 - Methane:  has only very weak London dispersion forces (lowest b.p. & 
m.p.) 
CHCl3 - Chloroform:  has dipole-dipole interaction (moderate b.p. & m.p.)
NH3 - Ammonia:  has hydrogen bonding and dipole-dipole interaction (high b.p. 
& m.p.)