Geology Reference
In-Depth Information
electronegative than hydrogen, it attracts the shared electrons to a greater
extent. In addition, the oxygen atom has a pair of lone orbitals. The overall
effect produces a molecule with a very strong dipole moment, having
distinct negative (O) and positive (H) ends. While there are several impor-
tant consequences, two will be considered here. Firstly, the positive H
atoms of one molecule are attracted towards the negative O atom in
adjacent molecules giving rise to hydrogen bonding. This has important
implications with respect to a number of physical properties, especially
those relating to thermal characteristics. Secondly, the large dipole moment
ensures that water is a very polar solvent.
Considering firstly the physical properties, water has much higher
freezing and melting points than would be expected for a molecule of
molecular weight 18. Water has high latent heats of evaporation and
fusion. Therefore, considerable energy is required to cause phase
changes, the energy being utilised in hydrogen-bond rupturing. More-
over, it has a high specific heat and is a good conductor of heat.
Consequently, heat transfer in water by advection and conduction gives
rise to uniform temperatures. The density of pure water exhibits anom-
alous behaviour. In ice, O atoms have 4 H atoms orientated about them
in a tetrahedral configuration. These units are packed together with a
hexagonal symmetry. At the freezing point, 0 1C, ice is less dense than
water. Heating breaks some hydrogen bonds and the molecules can
achieve slightly closer packing causing the density to increase. Thus, the
maximum density occurs at 4 1C because thermal expansion at higher
temperatures compensates for this compression effect. As will be dis-
cussed later, seawater differs in this respect. Thus, fresh ice floats on
water, which in part explains how rivers and lakes can freeze over, while
remaining liquid at depth. With respect to other properties, water has a
high surface tension that is manifest in stable droplet formation and has
a relatively low molecular viscosity and therefore is quite a mobile fluid.
Water is an excellent solvent. It is extremely polar and can dissolve a
wider range of solutes and in greater amounts than any other substance.
Water has a very high dielectric constant, a measure of the solvent's
ability to keep apart oppositely charged ions. The solvating characteristics
of individual ions influence their behaviour in solution, i.e., in terms of
hydration, hydrolysis and precipitation. Although water exhibits amp-
hoteric behaviour, electrolytic dissociation is quite small. Furthermore,
dissociation gives equal ion concentrations of both H
3
O
1
and OH
and
so pure water is neutral. The amphoteric behaviour enhances dissolution
of introduced particulate matter through surface hydrolysis reactions.
While the concept will be considered in detail below, the term salinity
(S%) is introduced here as a measure of the salt content of seawater, a
