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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?

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  • $\begingroup$ It would be better to ask this on Chemistry SE probably. Also it's denser as a gas, when molecular mass has importance. Whole diamond can be one molecule... $\endgroup$ – Mithoron Dec 16 '15 at 1:39
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    $\begingroup$ I'm voting to close this question as off-topic because it has nothing related to physics (okay, overlooking the fact that quantum & physical chemistry is build on physics). It is competent for Chem.SE . $\endgroup$ – user36790 Dec 16 '15 at 5:22
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The main reason is hydrogen bonding. Have a look at the following diagram ($^*$):

Hydrogen bonding.

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.

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    $\begingroup$ I don't think this is correct. Strong hydrogen bonding decreases the density by restricting the ability of the molecules to move. That's why ice is less dense than water - the strong hydrogen bonds enforce a more open structure. $\endgroup$ – John Rennie Dec 16 '15 at 6:52
  • $\begingroup$ There's serious misunderstanding about how hydrogen bonding works in this answer. Alcohol has bonds of similar strength but there's much less of them. $\endgroup$ – Mithoron Dec 16 '15 at 13:12
  • $\begingroup$ @Mithoron: in ethanol the hydrogen bonds are weaker because the O-C bond is less polarised, mainly due to electron pushing by the electron-rich ethyl group. That also explains why ethanol is an even weaker acid than water. I've edited my answer. $\endgroup$ – Gert Dec 16 '15 at 15:42
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Density is mass per volume, so if a molecule is larger, it has less mass per volume than an equal weight smaller molecule.

Also, you need to consider the distances between the molecules, which are influenced by various parameters: size and form of molecules, charge distribution, etc.

for comparison, it is easier to densely stack balls or squares than to stack irregular formed things, like crosses or chairs. Or imagine you want to pack pencils that have charged ends that repel each other on both ends?

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The density is defined as mass per unit volume.Ethanol has a less density relative to water.The boiling point of ethanol is lower than that of water due to its tendency of strong Hydrogen bonding.

If a molecule has a large size a large space will be occupied by the single molecule.So in a given amount of volume less no. Of molecules will be accommodated. As less no. Of molecules are present in the given amount of volume,so the mass per unit given volume will be less.Hence the density of the substance will be less.However,the density also depends on the arrangement of the atoms,ions in the substance.e.g. The density of the metal hydride is more than that of the metal due to its arrangement.

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