Polarity and Boiling Point:
The polarity of the molecules determines the forces of attraction
between the molecules in the liquid state. Polar molecules are
attracted by the opposite charge effect (the positive end of
one molecule is attracted to the negative end of another molecule.
Molecules have different degrees of polarity as determined by
the functional group present.
Principle: The greater the forces of attraction the higher
the boiling point or the greater the polarity the higher the
In the case of water, hydrogen bonding, which is a special
case of polar dipole forces exerts a very strong effect to keep
the molecules in a liquid state until a fairly high temperature
This is shown in the graphic on the left for a similar set
of molecules in Group VI of the periodic table. If water behaved
as a normal polar molecule it would have boiled at about - 100
C (shown in red). The other molecules are slightly polar and
show the increase in boiling point with molecular weight which
Instead, water boils at +100 C, which is very abnormal. The
major reason for this abnormal behavior is the strong attractions
afforded by the hydrogen bonds. It takes a lot more kinetic energy
in an increased temperature to break the hydrogen bonds to free
the water molecules as the gas.
More normal behavior is seen in dimethyl ether (CH3)2O
which has no hydrogen bonds possible.