1292 WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE
length of the other bond. The interdependence of the two OH
bonds is also observed in their different dissociation energies.
At 0K, the dissociation energies. At 0K, the dissociation
energy of H–OH is 117.8 kilocalories per mole or 5.11eV
whereas these respective values for H–O are 101.5 and 4.40.
Thus the energy of formation of a water molecule from the
individual atoms is 219.3 kilocalories per gram-mole.The Vapor Phase of WaterIn the vapor phase, water is mostly monomeric with the
occurrence of an occasional dimer and, perhaps, a very rare
trimer. At low pressure and low temperature the molecules do
not interact to any appreciable extent.The Solid Phases of WaterThe structure of the solid phases of water is relatively well
known compared to that of the liquid phase. Kamb (1972)
recognizes 13 distinct ice phases by including among the
distinctions certain “temperature dependent order-disorder”
phenomena. The transition points and the densities of these
phases are listed in Table 1.Ordinary IceOrdinary ice, ice I, also known as I h (I hexagonal ), is the only
solid phase stable at normal low pressures. The presence in
each water molecule of the two lone pairs of electrons on the
oxygen atom and the two protons in approximately tetrahe-
dral geometry create the opportunity for four intermolecular
hydrogen bonds resulting in a coordination number of four
for the ice structure. The arrangement of the oxygen atoms
is isomorphous with tridymite, one of the crystalline forms
of silica, and with wurtzite, a crystalline form of zinc sulfide.As shown by X-ray diffraction, ice I is a hexagonal lattice
with each oxygen atom surrounded tetrahedrally by four other
oxygen atoms at a distance of 2.76A. The oxygen atoms form
what many authors call “puckered” hexagonal rings. Each
ring layer is a mirror image of its neighbors. The hexagonal
arrangement of ice I gives rise to an open structure thereby
explaining the low density of ordinary ice, 0.9168 gcm^3 at
0 C, relative to its melt, 0.99987 gcm^3 at 0C.
The OH bond length has been determined to be 0.99
Å by studies of stretching vibrations. As Figure 3 illustrates,
a hydrogen atom occupies one of two possible positions on
each O–O line, amounting therefore to four hydrogen atoms
around each oxygen consistent with the tetrahedral spacing of
the oxygen atoms. These allowable positions are 1.05Å from
each molecule along the O–O axis. Early theoretical work by
Pauling based on the residual entropy in ice at 0C predicted
that over a sufficiently long time interval, the hydrogen atom
between the two oxygen atoms occupies each of these posi-
tions half of the time. This was later confirmed by neutron
diffraction experiments.The Polymorphs of IceThe other crystalline forms of ice can be produced at various
pressures and temperatures. The P–V–T surface illustrated
in Figure 4 defines the regions of stability for the various
polymorphs. Ice I c (I cubic ) has the same unambiguous half-
hydrogen distribution as I h , and thus the same configurational
entropy. At about 90 C its cubic crystalline structure readily
converts to the hexagonal form, ice I h , at a rate dependent on
its previous history.
Vitreous ice is so named since it is considered to be a glass.
This amorphous structure is formed by the condensation of
vapor below 150 C. Warming above this temperature trans-
forms it irreversibly to I c , and then to I h upon further heating.TABLE 1
The ice phasesLimits of temperature rangePhase Density g cm^3 Pressure kb Lower C Upper C Transition at upper limit
I 0.92 0 273 0 Melts
Ic. 0.92 0 150 (?) 90 inv. to I
vit. 0.94 0 273 (?) 120 inv. to Ic
II 1.21 3 273 30 trans. to III
III 1.15 3 90 (?) 20 melts
IV 1.29 5 40 (?) 25 (?) melts
V 1.28 5 150 (?) 10 (?) melts
VI 1.38 14 150 (?) 50 melts
VII 1.57 25 2 110 melts
VIII 1.63 25 273 2 trans. to VII
IX 1.19 3 273 150 trans. to III
X* ~1.4 — 273 150 (?) trans. to VI
XI* ~1.3 5 273 150 (?) trans. to VC023_004_r03.indd 1292C023_004_r03.indd 1292 11/18/2005 11:12:31 AM11/18/2005 11:12:31 AM