Alcohol and Water

Whisky shows some chemico-physical peculiarities we can't grasp intuitively. The following article tries to help you better understand the behaviour of whisky. And by that we don't mean the answer to the question why the fill level of some bottles always decreases with time. ;-)

An 3d image of ethanol and water on a molecular basis
Alcohol (ethanol) and water molecule
(Pictures by courtesy of Dr. Bernd Meynhardt/University of Kiel)

Whisky consists primarily of water and alcohol. The rest (<1%) are flavour substances and aromas. When we speak of alcohol in the context of food, we always mean ethyl alcohol, or ethanol for short. The chemical formula is C2H5OH. This mixture doesn't behave like pure water but also not like pure alcohol. The following article tries to illustrate the physical peculiarities of whisky.

Whisky producers take special care that no methyl alcohol (methanol, CH3OH) gets into the final product. The yeasts are bred in such a way that they don't produce methanol. Cutting off the foreshots during distillation provides double safety.

In the chart above is the ethanol molecule. The carbon atoms (C) are black, the oxygen atom (O) is red and the hydrogen atoms (H) are grey. To the right of the ethanol molecule you can see a water molecule (H2O).

A closer examination reveals that, except for the oxygen atom, the ethanol molecule is symmetrical to a perpendicular plane between the carbon atoms. Only one hydrogen atom has been replaced by an oxygen-hydrogen group (-OH group). In the sum total, the ethanol molecule is electrically neutral, but the -OH group causes local differences in electronegativity (so-called dipoles d+, d-). Normal water molecules have these dipoles, too, since they're also angled (109°). These dipoles on the ethanol and water molecules cause the formation of hydrogen bonds between the molecules.

The scematic of a Hydrogen Bond between ethanol and water
Hydrogen bonds between water and ethanol

Therefore the -OH group is hydrophilic (water-loving), while the rest of the ethanol, the C2H5 group, is lipophilic (fat-loving). Of course hydrogen bonds are always formed. Therefore to each dipole in the drawing above another dipole with the opposite electric charge is attached, be it a dipole of a water molecule or of an alcohol molecule.

Questions and Answers

Why does high-proof whisky heat up when diluted with water?

The -OH groups of the ethanol form hydrogen bonds with the water molecules, which produces heat of mixing.

Why do some whiskies become cloudy when diluted with water?

There are components in the whisky that are soluble in ethanol but hardly soluble in water. When you add water it takes some time until these components attach to the alcohol again and the whisky becomes clear again.

Why are some whiskies cooled before filtering?

Most chemical substances are better soluble at high temperatures than at low temperatures. During maturation, the whisky extracts some substances that can cause cloudiness from the cask wall. If you cool the whisky these substances coagulate and can be better absorbed by a filter.

Why do streaks form when whisky is mixed with water?

This is a physical rather than a chemical effect. Water and alcohol have a different density and refractivity, which leads to the formation of streaks. Once the water and the alcohol have mixed properly, no more streaks can be seen.

Why is it not possible to produce 100% pure ethanol by distillation?

Ethanol and water form an azeotrope (96% ethanol, 4% water), the boiling point of which is below the boiling point of pure ethanol (78.15°C/172.67°F at 1 bar (760mm) and 95.6% alcoholic content (weight)). When boiling an ethanol-water azeotrope, the boiling point of 100% pure ethanol can never be reached since the 96/4 mixture boils first. By adding benzene (warning: toxic!) 100% pure ethanol can be produced, since the benzene forms an azeotrope with the water, which can then be extracted. Nowadays molecular sieves provide a much healthier way to produce 100% pure ethanol.

Some physical data

alcoholic concentration

density
(20°C, kg/l)
refractive index relative to air
(20°C, sodium light)
relative viscosity to water  
(20°C)
0% 1)0.99821.33331.000
10%0.98191.33951.498
20%0.96871.34692.138
30%0.95391.35352.662
40%0.93521.35832.840
46%0.92271.36042.837
50%0.91391.36162.807
60%0.89111.36382.542
70%0.86761.36522.210
80%0.84361.36581.877
90%0.81801.36501.539
100% 2)0.78931.36141.201

1) pure water

2) pure alcohol