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... $\endgroup$– MithoronCommented Dec 16, 2015 at 1:39
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6$\begingroup$ 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 . $\endgroup$– user36790Commented Dec 16, 2015 at 5:22
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4 Answers
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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.
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1$\begingroup$ 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. $\endgroup$ Commented Dec 16, 2015 at 6:52
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$\begingroup$ 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. $\endgroup$– MithoronCommented Dec 16, 2015 at 13:12
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$\begingroup$ @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. $\endgroup$– GertCommented Dec 16, 2015 at 15:42
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The answer is related to the free volume of the material. This goes to the Gibbs Free energy of the materials. G=H-TS....the entropy(S) of the material and the enthalpy(H) of the material determines the ultimate lowest steady state energy of the material. Temperature(T, in degrees K) also plays a role. The hydrogen bonding state of water is a function of the distances and angles of the H2O structure. As water crystalizes(there are more than 10 crystal formations for water) this organizes the molecules into a 3D lattice of attractive and repulsive forces as the previous commenters are eluding to. For water with a 109degree angle between the two hydrogens and the oxygen atoms, this arrangement ends up in a structure that is less dense(less atoms/unit volume) than the more chaotic 'structure' of liquid water wherein each molecule has more 'degrees of freedom' to move within a given volume. I know this does not 'sound' like physics, but it is at the core of physics where Gibbs was working and elucidated to the world the mechanisms and laws of Free energy...which seems to be in the realm of this line of questioning.
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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.
answered Dec 16, 2015 at 17:42