Environmental Microbiology of Aquatic and Waste Systems

4 1 Nature, Properties, and Distribution of Water

one side of an oxygen atom, resulting in the V-shaped
structure mentioned above. On account of this, a water
molecule has a positive charge on the side where the
hydrogen atoms are, and a negative charge on the other
side, where the oxygen atom is (see Fig. 1.1). Oxygen
has a higher electronegativity than hydrogen. Because
oxygen has a slight negative charge while the hydrogen
has a slight positive charge, water is a polar molecule,
since there is a net negative charge toward the oxygen
end (the apex) of the V-shaped molecule and a net posi-
tive charge at the hydrogen end. Therefore, water
molecules tend to attract each other, because opposite
electrical charges attract. As will be seen below, this
molecular polarity causes water to be a powerful solvent
and is responsible for its strong surface tension. It is
also responsible for the high specific heat of water
(Pidwirny 2006 ; Chaplin 2009a; Anonymous 2009a).
The elements surrounding oxygen in the periodic
table, namely, nitrogen, fluorine, phosphorus, sulfur,
and chlorine, all combine with hydrogen to produce
gases under standard conditions. The reason that
oxygen hydride (water) forms a liquid is that it is more
electronegative than all of these elements (other than
fluorine).

1.1.2 The High Surface Tension of Water

and Capillarity in Plants

Water molecules as a whole have no net charge, but the

oxygen end has a slight negative charge (since the

electrons tend to stay on the side of the large oxygen

nucleus), and the hydrogen end has a slight positive

charge. For this reason, water is referred to as a polar

molecule because it has a positive end and a negative

end, analogous to the north and south poles of a bar

magnet. On account of this, the negative (oxygen) end

of one water molecule forms a slight bond with the

positive (hydrogen) end of another water molecule.

Because of the above, water has a high surface ten-

sion. In other words, water is adhesive and elastic, and

tends to aggregate in drops rather than spread out over

a surface as a thin film (see Fig. 1.2). This is a result of

water molecules attracting each other because of the

positive charge at one end and the negative charge at

the other. The strong surface tension also causes water

to stick to the sides of vertical structures despite gravi-

ty’s downward pull. Water’s high surface tension allows

for the formation of water droplets and waves, plants’

capillary of water (and dissolved nutrients) from their

roots to their leaves, and the movement of blood through

tiny vessels in the bodies of some animals (Sharp 2002 ;

Pidwirny 2006 ; Chaplin 2 009a).

1.1.3 The Three Physical States of Water,

and the Floatation of Ice

Water is unique in that it is the only natural substance

on earth that is found in all three physical states –

liquid, solid (ice), and gas (steam) – at the temperatures

normally found on earth. When the water molecule

H^105 o H

O

(^105) o
O H
H
105 o H
O
2
H
Fig. 1.1 The molecular structure of water (Modified from
Chaplin 2009a). Left: A water molecule showing the charges.
Right: Two water molecules attracting each other
Fig. 1.2 Attraction of
water molecules to create
high surface tension
(Copyright Michael
Pidwirny; http://www.our-planet-
earth.net. Reproduced with
permission from Pidwirny
2006 )
Note: In this illustration
the water film is two layers
thick