WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE 1293
Ice II though VII are formed at high pressure but can sub-
sequently exist at atmospheric pressure. Ice II is produced by a
transition from ice III at about 100 C. The oxygen atoms of
ice II, whose rhombohedral unit cell contains 12 water mole-
cules, are apparently in the same tetrahedral positions as those
in ice III, and the volume change of transition is very small but
the resulting spatial ordering yields a much lower dielectric
constant. Ices, II, III, V, and VI, all of which contain distorted
hydrogen bonds, have internal energies only several tenths of
a kilocalorie per gram-mole greater than ice I. In comparison,
the internal energy of liquid water at 0C is 1.88 kilocalories
per gram-mole greater than that of ice I.The crystal lattice of ice V is a monoclinic unit cell with
28 eater molecules. Ices IV, VII and VIII have extremely
high densities of 1.38, 1.57, and 1.63 gramscm^3 , respec-
tively. These high densities result from interpenetrating but
not interconnecting structures. According to Fletcher “dense
packing is achieved by placing water molecules upon two sep-
arate, completely hydrogen bonded four-coordinated lattices
which are allowed to interpenetrate so that the molecules of
one lattice occupy the cavities of the other.” Thermodynamic
criteria indicate that ice VI is probably metastale. Ice VII
forms at 22 to 200 kilobars; only one additional allotrope,
ice VIII, has been identified in this high pressure range.
Tables 2 and 3 from Kamb (1972) and Eisenberg (1969)
summarize the properties of the ice polymorphs.The Structure of Liquid WaterLiquid water is a highly structured liquid in which the tet-
rahedral condition observed in ice is still evident. The
structure of liquid water still remains a greater enigma than
highly complex molecules such as DNA and hemoglobin.
Nevertheless, newly acquired data of various properties
have increased the constraints operative in the hypothesis of
models for the liquid structure.HH CdTH SSC
TdT SGGHTST0 p pc T0p
pc
0T0TT0 0 T ∫ ∫
The subscript “pe” indicates phase change. Thermodynamic
constants for phase change are given in Table 4. (Figure after
Eisenberg and Kauzmann, 1969; data from Dorsey 1941).
An acceptable model must explain anomalous properties
of liquid water such as the following listed by Krundel and
Eliezer (1971):1) The density maximum at 4C above the melting
point.
2) A liquid specific heat approximately twice that of
the solid.
3) The extraordinary large heat of fusion.
4) The increase in the coefficient of thermal expan-
sion with increasing pressure (or, equivalently, the
decrease in compressibility with temperature rise)
between 0C and 45C compared with a decrease
in this coefficient under same conditions for
normal liquids.
5) The singular decrease in viscosity with increas-
ing pressure (in the temperature range 20C) to
a minimum near 1000kgcm^2 , above which the
normal increase is exhibited.
6) The exceptional dependence on pressure of the
dielectric constant, the self-diffusion coefficient
and other thermodynamic and transport properties.109.33’0‘2.752
D 11.011
109°6’
2.765
D 21.015109°52’ D‘ 2(^00) –
- 109°24’
- (1/2)0
- (1/2)0
FIGURE 3 Structural relationships in one tetrahedron
of the ice structure. The distances and angles are those
at 50 C. From Krindel (1971).Melting
pointBoiling
pointG,H [kJ.mol–1]Cp [J.mol–1°C–1]G–H 0CpH –^ H^0(S – So)100806040200–20–400 200 400 600–80–4004080120160200T (°K)S[J.mol- 1 °C
- 1]
FIGURE 4 Enthalpy, free energy, and isopiestic heat
capacity of H 2 O at 1 atmosphere pressure, calculated
from heat capacity measurements.C023_004_r03.indd 1293C023_004_r03.indd 1293 11/18/2005 11:12:31 AM11/18/2005 11:12:31 AM