Geology Reference
In-Depth Information
Box 4.1 The special properties of water
Water is so commonplace that one tends to overlook how
unusual its properties are in comparison with other liquids.
The water molecule is bent (Figures 4.1.1 and 7.9a,b), with
both hydrogen atoms occurring on the same side. As oxy-
gen attracts electrons more strongly than hydrogen, a
slight excess of electron (negative) charge δ − gathers at
the oxygen side of the molecule, leading to a correspond-
ing deficiency (a net positive charge δ +) on the two hydro-
gen atoms at the other side (Figure 4.1.1). This polarity is
responsible for most of the properties peculiar to water:
H
O
H
O
=
H
(a) Molecules of water in the liquid and solid states are
loosely bonded to each other by an electrostatic attrac-
tion between the negative end of one molecule and
the positive end of a neghbouring one. This hydrogen
bonding (Chapter 7) lies behind many of water's unique
properties.
(b) Hydrogen bonding gives liquid water a very high specific
heat, a high latent heat of vaporization (because water
molecules are more difficult to separate and disperse
into a gas phase), and an unusually large liquid-state
temperature range (100 °C). These thermal properties
of water give it a heat-exchange capacity of great cli-
matic significance on the Earth, as the moderating
effect of the oceans on seaboard climate illustrates. It
takes only 2.5 m depth of ocean water to match the
heat capacity of the entire atmospheric column above.
(c) Hydrogen bonding in the liquid state is also responsi-
ble for making liquid water denser than ice at tempera-
tures close to 0 °C (for the consequences of which,
see Box  2.2). Water therefore expands on freezing,
leading to the important erosional processes of frost-
shattering and frost-heaving (and the familiar fact that
ice floats on water). Water also has an unusually high
surface tension, resulting in strong capillary penetra-
tion of pore waters.
δ -
δ +
Figure 4.1.1 The geometry and polarity of the water
molecule.
(d) The polar nature of the water molecule makes water an
attractive environment for ions, and gives water its
unique capacity as a solvent of ionic compounds. Polar
water molecules, attracted by the electrostatic field of
each ion, cluster in a loose layer around it, aligning their
polarity according to the ion's charge. This association
of water molecules around a dissolved ion (an ion dipole
interaction - Chapter  7) is called hydration. The com-
bined electrostatic attraction of the polar water mole-
cules lowers the ion's potential energy and stabilizes it
in solution. Many ionic compounds remain hydrated
when they crystallize from solution, forming hydrated
salts such as the mineral gypsum, CaSO 4 2H 2 O. Water
is thus the prime example of a polar solvent .
(e) Conversely, water is much less effective as a solvent
of non-polar organic substances.
Ways of expressing the concentrations
of major constituents
that it is rarely of interest). There are several ways of
expressing concentration. The obvious one is to state
the mass of each solute per unit volume of solution, 1 in
units such as g dm −3 , mg dm −3 or μg dm −3 . But in consid-
ering the properties of the solution and the reactions
Solutions
The composition of a solution can be expressed by stat-
ing the concentration of each of the solute species pre-
sent (the amount of solvent present is so nearly constant
The SI unit of volume used in solution chemistry is the cubic
decimetre (dm 3 ), which is equal to a litre (L). See Appendix A.
1
 
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