1.2 Importance and Uses of Water 7
impurities are absent. Any electrical properties obser-
vable in water are from the ions of mineral salts and
carbon dioxide dissolved in it. Water does self-ionize
where two water molecules become one hydroxide
anion and one hydronium cation, but not enough to
carry sufficient electric current to do any
work or harm for most operations. In pure water,
sensitive equipment can detect a very slight electrical
conductivity of 0.055 mS/cm at 25°C. Water can also
be electrolyzed into oxygen and hydrogen gases but in
the absence of dissolved ions, this is a very slow
process since very little current is conducted.
1.1.7 The High Chemical Reactiveness
of Water
Water is very chemically reactive, reacting with certain
metals and metal oxides to form bases, and with cer-
tain oxides of nonmetals to form acids. It reacts with
certain organic compounds to form a variety of products,
e.g., alcohols from alkenes. Although, as seen above,
completely pure water is a poor conductor of elec-
tricity, it is a much better conductor than most other pure
liquids because of its self-ionization, i.e., the ability of two
water molecules to react to form a hydroxide ion, OH-,
and a hydronium ion, H 3 O+. Its polarity and ionization
are both due to the high dielectric constant of water.
1.1.8 The pH of Water
Water in a pure state has a neutral pH. As a result, pure
water is neither acidic nor basic. Water changes its
pH when substances are dissolved in it. Rain has a
naturally acidic pH of about 5.6 because it contains
dissolved carbon dioxide and sulfur dioxide.1.1.9 The High Solvent Power of WaterBecause water is a polar compound, it is a good solvent.
Water is a very strong solvent; it is referred to as the
universal solvent because it dissolves many types of
substances. The substances that will mix well and dis-
solve in water (e.g., salts) are known as “hydrophilic”
(water-loving) substances, and those that do not mix
well with water (e.g., fats and oils) are known as
“hydrophobic” (water-fearing) substances. The ability
of a substance to dissolve in water is determined by
whether or not the substance can match or better the
strong attractive forces that water molecules generate
between other water molecules. If a substance has
properties that do not allow it to overcome these strong
intermolecular forces, the molecules are “pushed out”
from among the water and do not dissolve. Water is
able to dissolve a large number of different chemical
compounds and this feature enables it to carry dissolved
nutrients in runoff, infiltration, groundwater flow, and
living organisms.1.2 Importance and Uses of Water
1.2.1 Composition of Biological ObjectsMost living things consist of a large percentage of water:
In humans, for example, it constitutes about 92% of
blood plasma, about 80% of muscle tissue, and about
60% of red blood cells. This is because water is required
for the biochemical reactions taking place in them.1.2.2 Drinking by Man and Animals
and Domestic UseAll living things need water for their metabolism.
Water is absolutely essential for survival. A person
may survive for a month without food, but only about
a week without water. In order to be clean enough for
human consumption, water usually has to be treated in
some way to remove harmful microorganisms and
chemicals. In the home, water is used for cooking,
cleaning, and disposal of wastes as sewage.2HOHOOH 23
+−+
Table 1.2 Density of water molecules at various temperatures
(Modified from SIMetric.co.uk; http://www.simetric.co.uk/si_
water.htm; Anonymous 2 009b)
Temperature (°C) Density (gm/cm^3 )
0 (solid) 0.9150
0 (liquid) 0.9999
4 1.0000
20 0.9982
40 0.9922
60 0.9832
80 0.9718
100 (gas) 0.0006