Ethyl alcohol (ethanol) is less dense than water, and it's boiling point is lower. But this molecule is more complex and bigger than the simple H2O. How can a substance with a higher molecular size have less density?
[Physics] Why is alcohol less dense than water
densitymoleculesphysical-chemistrywater
Best Answer
The main reason is hydrogen bonding. Have a look at the following diagram ($^*$):
Hydrogen and oxygen have significantly different values of electronegavity (the tendency of an atom to attract electrons) which causes molecular bonds between them to be permanently polarised. The oxygen atoms (shown in red) in the $\mathrm{H_2O}$ molecule have a permanent negative charge, the hydrogen atoms (shown in white) a permanent positive charge. Such molecules are called permanent dipoles. Electrostatic attractions between these charges are shown by thin dotted lines.
These electrostatic attractions decrease the average distance between the molecules, thereby increasing density.
They also make it harder for molecules to leave the liquid phase, resulting in higher boiling point.
Ethanol too shows some hydrogen bonding, as it is also a permanent dipole weaker than water. There are also fewer of them.
In addition to permanent dipoles, some molecules are also polarisable: when the electron clouds that make up the bonds in the molecule clash with those of anther molecule during a collision, electrostatic repulsion between them causes transient polarisation. This tends to increase boiling point a little. More complex, longer molecules are more polarisable and tend to have higher boiling points.
$^*$: the diagram is for hexagonal ice but the principle of hydrogen bonding in liquid water is the same.