Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE 1291

no longer thermodynamically stable. Specifically, these are

chemical conditions which subject the minerals to hydroly-

sis, oxidation, and reaction with aqueous carbon dioxide.

The minerals also react with weak organic acids, which can

be considered biological weathering, since these acids result

from the microbially mediated decay of biomass. Whereas

surface runoff streams erode, transport and eventually deposit

much of their suspended weathering products in river beds on

the ocean, the dissolved species generally remain in solution.

Global Ice

Most of the earth’s fresh water exists as ice in glaciers and in

ice sheets, which together comprise about 2% of the earth’s

total water and which cover about 10% of the earth’s land

masses with thicknesses of up to several thousand feet.

These solid forms may well be the most potent of the agen-

cies that have changed and shaped the surface of the earth,

according to King. Their mass causes the land to sink and

their physical force performs erosion, transport, and deposi-

tion of mineral matter.

Ocean Currents

Water eventually returns to the sea with the wastes of nature

such as dissolved minerals and suspended matter and,

ever more significantly, also the wastes of man. To the sea

belongs 97% of the planet’s water. The sea is not a stagnant

reservoir; instead it contains large “rivers” such as the Gulf

Stream, which convey huge masses of water and perhaps

more important great quantities of heat. For example, if the

temperature difference between the warm current of the Gulf

Stream and the cold receiving water is 20F then each cubic

mile of water can transfer about 2 
 20 14 BTU.

Determinants of Physical–Chemical Properties

The properties of water and its interactions with other sub-

stances are in a class by themselves. The chief contributant

to the anomalous physical characteristics of water and to

many of its chemical properties is its powerful intermolecu-

lar forces, with which it attracts not only its own molecules,

but also those of many other substances. The main intermo-

lecular attractive force of water is hydrogen bonding which

in turn results from the nature of the chemical bonds holding

the atoms of the individual water molecule together.

Hydrogen Bonds

Hydrogen bonding is a special case of dipole-dipole inter-

action resulting from the molecular polarization when the

hydrogen atom is covalently bonded to a highly electronega-

tive atom. Such a condition produces a large dipole moment

by displacing the electrons of the shared bond, which

includes the sole electron of the hydrogen atom, toward the

negative atom and gives a strong partial ionogenicity to the

bond and a strong positive character to the hydrogen atom.

This leaves an intense unshielded electric field on the other

side of the hydrogen nucleus which can attract the unshared

electrons of the negative atoms of other molecules.

STRUCTURAL AND PHYSICAL PROPERTIES

Electronic Structure of Water

A model of the single isolated water molecule with its elec-

tronic cloud is shown in Figure 2. The three nuclei are sur-

rounded by ten electrons, with the two 1s electrons of the

oxygen atom confined to the vicinity of its nucleus. The other

eight electrons make up the four orbitals which point to the

vertices of a somewhat distorted tetrahedron. Two of the four

orbitals are directed along the OH bonds, and the so-called

lone pair electrons constitute the remaining two orbitals which

project above and below the H–O–H molecular plane. These

lone pair electrons provide regions of negative electrification,

which can attract the positively charged nuclei of hydrogen

atoms in other water molecules thereby giving rise to hydro-

gen bonds. The charge distribution within the molecule results

in the relatively large dipole moment of 1.84 
 10 ^18 esu.

Molecular Structure of Water

The planar H–O–H angle in the water molecule is 10431,

and the OH bond length is 0.9572 Angstrom. The symmetric

group is C 2 v : it has twofold axis of rotational symmetry, C 2 ,

(the line bisecting the H–O–H angel) and a plane of reflec-

tion, v passing through this axis and normal to the plane of

the molecule.

Precise calculations indicate that the average electron dis-

tributions forming the covalent OH bonds do not lie directly

along the lines form the oxygen nucleus to the hydrogen

nuclei, but are bent inward. Furthermore, the normal mode

analysis of vibrational spectrum of the two OH bonds shows

that the equilibrium length of one bond is dependent on the

+

+

FIGURE 2 Single isolated water molecule with

associated electronic cloud. Note the two regions

of negative electrification which project above and

below the H–O–H plane. From Horne (1970).

C023_004_r03.indd 1291C023_004_r03.indd 1291 11/18/2005 11:12:31 AM11/18/2005 11:12:31 AM