1.1 Physical and Chemical Properties of Water and Their Consequences 5
makes a physical phase change, its molecules arrange
themselves in distinctly different patterns. In ice,
the mole cules are highly organized; in liquid water, they
are semi-ordered, while the molecules are randomly
arranged in water vapor (see Fig. 1.3).
The molecular arrangement taken by ice (the solid
form of the water molecule) leads to an increase in volume
and a decrease in density. Expansion of the water mole-
cule at freezing allows ice to float on top of liquid water.
Water molecules exist in liquid form over an impor-
tant range of temperature from 0°C to 100°C. This
range allows water molecules to exist as liquid in most
places on earth (Anonymous 2009b).
1.1.4 The Thermal Properties of Water
and Their Effect on Climate
Because of hydrogen bonding between water
molecules, the latent heats of fusion and of evaporation
and the heat capacity of water are all unusually high.
For these reasons, water serves both as a heat-transfer
medium (e.g., ice for cooling and steam for heating)
and as a temperature regulator (the water in lakes and
oceans helps regulate the climate). Latent heat, also
called heat of transformation, is the heat given up or
absorbed by a unit mass of a substance as it changes
from a solid to a liquid, from a liquid to a gas, or the
reverse of either of these changes. Incorporated in
the changes of state are massive amounts of heat
exchange. This feature plays an important role in the
redistri bution of heat energy in the earth’s atmosphere.
In terms of heat being transferred into the atmosphere,
approximately 75% of this process is accomplished by
the evaporation and condensation of water (see The
Hydrological Cycle below: Fig. 1.4).
Water has the second highest specific heat capacity
of any known chemical compound, after ammonia.
Specific heat is the amount of energy required to change
the temperature of a substance. Because water has a
high specific heat, it can absorb large amounts of heat
energy before it begins to get hot. It also means that
water releases heat energy slowly when situations cause
it to cool. Water’s high specific heat allows for the
moderation of the Earth’s climate, through the heat in
oceans, and helps organisms regulate their body tem-
perature more effectively. This explains why water is
used for cooling, say in automobile radiators, and why
the temperature change between seasons is gradual
rather than sudden, especially near the oceans.
Finally, water conducts heat more easily than any
liquid except mercury. This fact causes large bodies of
liquid water like lakes and oceans to have essentially a
uniform vertical temperature profile (Table 1.1).1.1.5 The Change in the Nature of Water
at Different Temperatures and Lake
TemperaturesAn unusual property of water is that the solid state, ice,
is not as dense as liquid water because of the geometry
of the hydrogen bonds (see above) which are formed
only at lower temperatures. For almost all other
substances the solid form has a greater density than
the liquid form. On account of this unusual property
the solid form, ice, floats on its liquid form. Freshwater
at standard atmospheric pressure is most dense at
3.98°C, and will sink by convection as it cools to thatFig. 1.3 Arrangement of molecules in steam, liquid water, and
ice (Copyright Michael Pidwirny; http://www.our-planet-earth.net.
Reproduced with permission from Pidwirny 2006 ). Note the highly
organized molecules in ice, the semi-ordered arrangement in liquid
water, and the random molecular structure in water vapor