Geoscience Reference
In-Depth Information
it pass! The ability to act as an efficient solvent
comes through water molecules disassociating from
each other and being able to surround charged
compounds contained within them. As described
earlier, the ability of water to act as an efficient
solvent allows us to use it for washing, the disposal
of pollutants, and also allows nutrients to pass from
the soil to a plant.
In water's solid state (i.e. ice) the hydrogen bonds
become rigid and a three-dimensional crystalline
structure forms. An unusual property of water is
that the solid form has a lower density than the
liquid form, something that is rare in other com-
pounds. This property has profound implications for
the world we live in as it means that ice floats on
water. More importantly for aquatic life it means
that water freezes from the top down rather the
other way around. If water froze from the bottom
up, then aquatic flora and fauna would be forced
upwards as the water froze and eventually end up
stranded on the surface of a pond, river or sea. As
it is the flora and fauna are able to survive under-
neath the ice in liquid water. The maximum density
of water actually occurs at around 4°C (see Figure
1.3) so that still bodies of water such as lakes and
ponds will display thermal stratification, with water
close to 4°C sinking to the bottom.
Water requires a large amount of energy to heat
it up. This can be assessed through the
specific heat
capacity
, which is the amount of energy required
to raise the temperature of a substance by a single
degree. Water has a high specific heat capacity rela-
tive to other substances (Table 1.1). It requires
4,200 joules of energy to raise the temperature of
1 kilogram of liquid water (approximately 1 litre)
by a single degree. In contrast dry soil has a specific
heat capacity of around 1.1 kJ/kg/K (it varies accord-
ing to mineral make up and organic content) and
alcohol 0.7 kJ/kg/K. Heating causes the movement
of water molecules and that movement requires
the breaking of the hydrogen bonds linking them.
The large amount of energy required to break the
hydrogen bonds in water gives it such a high specific
heat capacity.
We can see evidence of water's high specific heat
capacity in bathing waters away from the tropics.
It is common for sea temperatures to be much lower
than air temperatures in high summer since the
water is absorbing all the solar radiation and heat-
ing up very slowly. In contrast the water tempera-
ture also decreases slowly, leading to the sea often
being warmer than the air during autumn and
winter. As the water cools down it starts to release
the energy that it absorbed as it heated up. Conse-
quently for every drop in temperature of 1°C a
single kilogram of water releases 4.2 kJ of energy
into the atmosphere. It is this that makes water
a climate ameliorator. During the summer months
a water body will absorb large amounts of energy
as it slowly warms up; in an area without a water
body, that energy would heat the earth much
quicker (i.e. dry soil in Table 1.1) and consequently
air temperatures would be higher. In the winter the
energy is slowly released from the water as it cools
down and is available for heating the atmosphere
1.02
1
0.98
0.96
0.94
Table 1.1
Specific heat capacity of various
substances
0.92
0.9
Substance
Specific heat capacity
(kJ/kg/K)
-10
0
10
20
30
40
50
Temperature (°C)
Water
4.2
Dry soil
1.1
Figure 1.3
The density of water with temperature. The
broken line shows the maximum density of water at
3.98°C.
Ethanol (alcohol)
0.7
Iron
0.44
