Page 70 - Materials Science and Engineering An Introduction

42 • Chapter 2 / Atomic Structure and Interatomic Bonding

M A T E R I A L S O F I M P O R T A N C E

Water (Its Volume Expansion Upon Freezing)

U pon freezing (i.e., transforming from a liquid relatively open structure—that is, the molecules are
not closely packed together—and as a result, the
to a solid upon cooling), most substances expe-
rience an increase in density (or, correspondingly, a density is comparatively low. Upon melting, this
decrease in volume). One exception is water, which structure is partially destroyed, such that the water
exhibits the anomalous and familiar expansion upon molecules become more closely packed together
freezing—approximately 9 volume percent expan- (Figure 2.24b)—at room temperature, the average
sion. This behavior may be explained on the basis number of nearest-neighbor water molecules has
of hydrogen bonding. Each H 2 O molecule has two increased to approximately 4.5; this leads to an in-
hydrogen atoms that can bond to oxygen atoms; crease in density.
in addition, its single O atom can bond to two Consequences of this anomalous freezing phe-
hydrogen atoms of other H 2 O molecules. Thus, for nomenon are familiar; it explains why icebergs float;
solid ice, each water molecule participates in four why, in cold climates, it is necessary to add antifreeze
hydrogen bonds, as shown in the three-dimensional to an automobile’s cooling system (to keep the engine
schematic of Figure 2.24a; here, hydrogen bonds block from cracking); and why freeze–thaw cycles
are denoted by dashed lines, and each water mol- break up the pavement in streets and cause potholes
ecule has 4 nearest-neighbor molecules. This is a to form.

H H
Hydrogen bond O

H
H
O H
O H
O
H
O
H
H
(a)

H
H
H
O
H H
O H
H
H
Photography by S. Tanner H O H H H O H H H O H
O
H
O

O H H H O O
A watering can that ruptured along a side panel— (b)
bottom panel seam. Water that was left in the can
during a cold late-autumn night expanded as it froze Figure 2.24 The arrangement of water (H 2 O)
and caused the rupture. molecules in (a) solid ice and (b) liquid water.