bound into large molecules in which each oxygen atom is connected through
hydrogen bonds to four other oxygen atoms as shown in Figure 30.
Figure 30. Study of Ice CrystalThis rather wide open structure accounts for the low density of ice. As heat
is applied and melting begins, this structure begins to collapse but not all the
hydrogen bonds are broken. The collapsing increases the density of the water, but
the remaining bonds keep the structure from completely collapsing. As heat is
absorbed, the kinetic energy of the molecules breaks more of these bonds as the
temperature rises from 0° to 4°C. At the same time this added kinetic energy tends
to distribute the molecules farther apart. At 4°C these opposing forces are in
balance—thus the greatest density. Above 4°C the increasing molecular motion
again causes a decrease in density since it is the dominate force and offsets the
breaking of any more hydrogen bonds.
This behavior of water can be explained by studying the water molecule
itself. The water molecule is composed of two hydrogen atoms bonded by a polar
covalent bond to one oxygen atom.
Because of the polar nature of the bond, the molecule exhibits the charges
shown in the above drawing. It is this polar charge that causes the polar bonding
discussed in Chapter 3 as the hydrogen bond. This bonding is stronger than the
usual molecular attraction called van der Waals forces or dipole-dipole
attractions.