Ask A Scientist© General Science Archive

name         fehmi
status       student
age          20s
Question -   Why is water's volume smallest at 4 Celsius?

fehmi,

At temperatures above absolute zero (0 K, -273 C), all molecules are in
incessant random motion. Such movements rather interfere with their
ability to hold on to each other. For example, at the boiling point of
water (100 C) the molecules are able to tear away from each other and
move from a liquid phase into the gaseous phase that you recognize as
vapor.

As the temperature of liquid water is reduced, random movements of the
water molecules are likewise reduced. At 4 C, molecular motion has been
diminished to the point where the hydrogen bonds that held them together
in the liquid phase are beginning to settle into an optimum (best)
arrangement that will ultimately bind them together in the ice
latticework at 0 C. From about 4 C to water's freezing point at 0 C, the
molecules are no longer able to so easily slip past each other as they
did in the liquid phase. They begin to really "feel" the intermolecular
attraction of hydrogen bonding which occurs between the hydrogen atoms
of one water molecule and the oxygen atoms of nearby molecules. Thus,
they begin to take on an orderly crystalline arrangement that we
recognize as ice. The ice latticework simply takes up more space than
the slightly more compact and disordered liquid state. Water expands
when it freezes.

Water is unusual in this regard. Most substances shrink when they pass
from the liquid to solid state.

Check introductory chemistry textbooks under the heading of "hydrogen
bond" for a more detailed explanation.

Regards,

ProfHoff
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