Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE 1297

CHEMICAL PROPERTIES OF NATURAL WATERS

Natural waters acquire their chemical characteristics by dis-

solution of and by chemical reactions with solids, liquids,

and gases with which they have come into contact during

the various parts of the hydrologic cycle. Waters vary in

their chemical composition, but these variations are at

least partially understandable if the environmental history

of the water and the chemical reactions of the rock-water-

atmosphere systems are considered. Dissolved mineral matter

is provided by the crustal materials of the earth; water dis-

integrates and dissolves mineral rocks by weathering. Gases

and volatile substances participate in these processes. As

a first approximation, seawater may be interpreted as the

result of a gigantic acid-base titration: acids of volcanoes

(sulfur dioxide, hydrochloric acid, hydrogen sulfide) react-

ing with bases of rocks (oxides, carbonates, silicates). The

composition of fresh water similarly may be represented as

resulting from the interaction of the carbon dioxide of the

atmosphere with mineral rocks. The earth may be regarded

as a heat engine which extracts energy from solar radiation

in order to drive winds, ocean currents, hydrogeochemical

cycles (cycles of water and rocks) and life cycles. Water,

acting as a transport medium as well as a chemical reagent,

gradually converts primary rocks into soil, sediments, and

sedimentary rocks (Figure 9).

Weathering Processes

Dissolution reactions due to weathering take place because

many constituents of the earth’s crust are thermodynamically

unstable in the presence of water and the atmosphere. Most

important among the weathering reactions is the incongruent

–20^020406080100

Temperature [°C]

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Pressure [bar]

0

0.01

0.02

0.03

0.04

0.05

–10 0 10 20

VAPOR

LIQUID

WATER

ICE I

TRIPLE

POINT

FIGURE 6 The vapor pressure of ice and liquid wa-

ter as a function of temperature. The inset shows the

vicinity of the triple point. (Data from Eisenberg and

Kauzmann, 1969.)

(^0204060)
60
70
80
80
90
100
Temperature [°C]
50
Dielectric Constant
FIGURE 7 The dielectric constant of water as a
function of temperature. (Data from Malmberg and
Maryott, 1956.)
0
0
0.5
1.0
1.5
2.0
20 40 60 80 100
Temperature [°C]
Viscosity [centipoise]
FIGURE 8 The shear viscosity of water as a func-
tion of temperature. (1 centipoise = 10−3 N s m−2).
(Data from Stokes and Millis, 1965.)
where t is the shear stress (force in the x -direction per unit
area normal to z) to maintain the velocity gradient d v x dz in
the z-direction, in the fluid of viscosity m. The viscosity of
water approximately doubles in the range between 25C and
0 C (Figure 8).
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