Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 66

while also considering their interactions. The chapter concludes with a discussion of the
anthropogenic effects on surface water and groundwater.

3.2 Water Basics
This section begins with the basic chemistry and properties of water. The focus then turns
to water's abundance and distribution on Earth, with special attention given to the work
water accomplishes as it moves through its global cycle. We finish with a discussion of
potable water.

3.2.1 Basic Water Chemistry

A water molecule is composed of two hydrogen atoms bonded with one
atom of oxygen. This bonding ratio gives water a chemical formula of
H 2 O. The spatial arrangement of the atoms within each water molecule
is shown in Figure 3.1 (Hill et al. 1993). The hydrogen atoms are attached
to each side of the oxygen atom, and this configuration results in a polar-
ity or directional charge. The water molecule is positively charged at the
hydrogen end and negatively charged on the opposite side where the
oxygen atom is located. Since opposite electrical charges attract, water
molecules have an affinity for each other, and, in its liquid state,
water forms the linear configuration as shown in Figure 3.2 (Hill
et al. 1993).
The angular arrangement of the water molecule shown in
Figure 3.1 results from the properties of its chemical bonds.
Hydrogen atoms are attached to the oxygen atom in a covalent
bond (Figure 3.3), with each black dot representing an electron.
Note the circled pairs of electrons on the two sides opposite the
hydrogen atoms. The pairing of electrons creates a slightly greater
negative charge, which acts to push the two hydrogen atoms closer.
This push results in a slightly bent tetrahedral molecule with an
angle of approximately 107.5° (Figure 3.4).
The polarity of a water molecule is significant, because it permits
water to

H +

-

O

H +

FIGURE 3.1
Water molecule.

H

+ H H

+ +

-

O -

O -

O

H

+ H +

H +

FIGURE 3.2
Electrical attraction of
water molecules.

O H

H

FIGURE 3.3
Water molecule showing
electron arrangement.

• Stick together with other water molecules, as in a drop of water.
• Interact with other polar molecules—this is why other sub-
stances are able to dissolve in water. Water is known as the
“universal solvent” because it dissolves more substances than
any other liquid.
• Interact with electricity.

H
δ+

δ-

107.5° O

δ-

H
δ+

Some important chemical properties of water include

FIGURE 3.4
Angular configuration
and polarity of the water
molecule.

• It freezes at 32°F or 0°C.
• It boils at 212°F or 100°C at mean sea level.