Geoscience Reference
In-Depth Information
Latent Heat
Let's review the concept of latent heat and how it relates to the
phases of water. To do so, view the animation
Latent Heat
. This
feature nicely illustrates the various phases of water and the way
that latent heat is either absorbed or released. After you view the
animation, be sure to answer the questions at the end to test
your understanding of this concept.
increasing amounts of nitrogen and oxygen molecules go into solu-
tion. During the process of freezing, 80 calories (cal) of heat energy
are released for every gram of water as the
latent heat of freezing
.
Have you ever wondered why ice floats on liquid water,
even though it is solid? The reason is that ice is actually less
dense than liquid water. This decreased density occurs because
as more hydrogen bonds form through freezing, the medium
expands and volume increases. Water can increase by 9% in
volume, which is why water pipes sometimes break in homes
during the winter. As you will see in Chapters 14 (Weather-
ing and Mass Movement) and 17 (Glacial Geomorphology:
Processes and Landforms), the ability of water to expand and
contract has had a profound impact on shaping Earth's surface.
When heat energy is applied to and absorbed by ice at 0°C
(32°F), the motion of the water molecules increases and some
of the hydrogen bonds begin to break. As a result, the ice melts
and becomes liquid water, which is a noncompressible fluid that
conforms to the shape of its container (such as a glass). During
this transformation, 80 cal of latent heat are absorbed as the
latent heat of melting
to change 1 gram (g) of ice to 1 g of water.
At this point the water molecules are still bound together, but
the bonds are sufficiently weak that water flows.
Now let's examine what happens when liquid water changes
to water vapor. This process begins at room temperature when
added energy causes hydrogen bonds to further loosen, resulting
in the liberation of some water molecules to the vapor phase in
the process of
evaporation
. The added heat is referred to as the
latent heat of vaporization
and amounts to 600 cal for every 1 g
of water. If the temperature of water increases to 100°C (212°F),
the tremendous amount of added energy causes all molecular
bonds to break, and all water molecules thus move into the at-
mosphere as vapor. Conversely, when water vapor changes back
to liquid form through the process of
condensation
, 600 cal of
energy are released as the
latent heat of condensation
.
In somewhat rare circumstances, it is possible for water
to change directly from the ice phase to vapor in a process
called
sublimation
. This transformation occurs if the tem-
perature of ice rapidly increases from 0°C (32°F) to 100°C
(212°F). Under these conditions, 680 cal of energy known as
the
latent heat of sublimation
are added to the medium. This
amount of energy is derived from the addition of the 600 cal
associated with vaporization
plus
the 80 cal absorbed when
ice changes to liquid water. In other words, the energy associ-
ated with both the melting and vaporization processes must be
accounted for. Conversely, water can change directly from the
vapor phase to ice through the process of
deposition
. When
this transformation occurs, 680 cal of latent heat are released,
which reflects the addition of the energy released by
both
con-
densation and freezing.
The Hydrosphere
and the Hydrologic Cycle
The previous discussion implies that water is stored in
various places on Earth such as the atmosphere, lakes, and
oceans. Overall, the total water realm of Earth is known as the
hydrosphere
. Within the hydrosphere, approximately 97.2%
of water is stored in the oceans as salt water (Figure 7.3). The
remaining 2.8% is largely freshwater, most of which is stored
in massive polar ice sheets and smaller mountain glaciers.
This frozen water accounts for about 2.15% of the total global
water volume. Approximately 0.63% is stored within Earth as
groundwater.
What about the remaining 0.02% of hydrosphere water?
Although this amount is analogous to
only one drop
of liquid in a
Evaporation
The process through which water changes from
the liquid to vapor phase.
Sublimation
The process through which water changes
directly from ice to the vapor phase.
Condensation
The process through which water changes
from the vapor to liquid phase.
Deposition
The process by which water vapor changes
directly to ice.
The water realm on Earth.
Hydrosphere
