Ask A Scientist Chemistry Archive

name         Debbie K.
status       educator
age          40s

Question -   When alcohol and water combine to take up less volume
(than all water), is it because the water molecules compact into the
alcohol molecules due to the alcohol molecule being more spacious? If
not, what causes the decrease in volume?
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There are many examples of liquids that have a negative volume of mixing.
The reasons are not always evident, and are frequently restatements of "It
happens because it happens." rather than providing a fundamental explanation
that can be independently tested experimentally. In the case of ethanol and
water, the "reason" is that liquid water has a rather "open" structure due
to hydrogen bonding. The same is not so true of ethanol. So when the two are
mixed, the ethanol interferes with this "open" structure of water and causes
it to "collapse", and hence reduce the volume relative to the sum of the
volumes of the components. A related property of water is the minimum in the
molar volume at about 4C. If a water soluble solid has an "open" structure
that can be made more compact by dissolving, one would expect a similar
result. It is difficult to predict if and when it will happen because it
could also depend upon the temperature and the pressure at which the
components are mixed. That is why I choose to say the explanation is "It
happens because it happens."

Vince Calder
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