When do molecules become polar?
A molecule is polar when its structure contains polarity, i.e. significant charge separation between bound atoms. This is a necessary but not a sufficient condition.
A polar molecule contains functional groups that enforce charge separation.....Alcohols are the classic polar group.....and we may represent this polarity by a diagram such as
While an INDIVIDUAL bond may be polar, it is the GEOMETRIC SUM of ALL the bond dipoles that determines MOLECULAR POLARITY. Methylene chloride,
Of course the intermolecular hydrogen bonding that results is a special case of bond polarity, but it might also occur in non-hydrogen bonding molecules such as
This bond polarity gives rise to greater intermolecular interaction, and decreased volatility.