Upon
freezing (i.e., transforming from a liquid to a solid upon cooling), most
substances experience an increase in density (or, correspondingly, a decrease
in volume). One exception is water, which exhibits the anomalous and familiar
expansion upon freezing approximately 9 volume percent expansion. This decrease
in density makes ice lighter than water that’ why ice floats on water.
This
behavior may be explained on the basis of hydrogen bonding. Each H2Omolecule
has two hydrogen atoms that can bond to oxygen atoms; in addition, its single O
atom can bond to two hydrogen atoms of other H2Omolecules. Thus, for
solid ice, each water molecule participates in four hydrogen bonds as shown in
the three-dimensional schematic of figure a, here hydrogen bonds are denoted by
dashed lines, and each water molecule has 4 nearest-neighbor molecules. This is
a relatively open structure—i.e., the molecules are not closely packed
together—and, as a result, the density is comparatively low. Upon melting, this
structure is partially destroyed, such that the water molecules become more
closely packed together (figure b) at room temperature the average number of
nearest-neighbor water molecules has increased to approximately 4.5; this leads
to an increase in density. Consequences of this anomalous freezing phenomenon
are familiar. This phenomenon explains why icebergs float, why, in cold
climates, it is necessary to add antifreeze to an automobile’s cooling system
(to keep the engine block from cracking), and why freeze-thaw cycles break up the
pavement in streets and cause potholes to form.
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