Geoscience Reference
In-Depth Information
molecules if they are to be formed or tightened. When bonds are
loosened, energy is transformed from sensible heat to latent heat,
which has a net cooling effect because less sensible heat is sensed.
In contrast, when bonds are strengthened, energy is converted
from latent heat to sensible heat, which has a net warming effect.
The thermal properties of water are important because they
strongly influence weather patterns on Earth, providing over 30%
of the energy required to drive atmospheric processes. For example,
much of the energy contained within a thunderstorm is produced
when latent heat is released because water changes rapidly from gas
to liquid form (this will be discussed in more detail later). This sec-
tion of the chapter focuses on the way that water changes physical
states and the amount of latent heat energy released or absorbed dur-
ing these transformations. As you follow this discussion, refer fre-
quently to Figure 7.2, which shows the various pathways that water
follows and the amount of latent heat that is absorbed or released.
Let's begin the examination of water phases and latent heat
transfers by considering what happens when liquid water turns to
ice in the process of
freezing
. Think of this process as one in which
progressively more hydrogen bonds develop and tighten. It actu-
ally starts when water cools to a temperature below 4°C (39°F) be-
cause the motion of water molecules begins to slow considerably,
which allows hydrogen bonds to strengthen. When liquid water
cools down to 0°C (32°F), the motion of the water molecules slows
even further, still more hydrogen bonds develop and tighten, and ice
forms. The exact temperature at which water crystallizes to form ice
depends on the air pressure. Water has to cool a bit more to freeze
at higher air pressure, for example, because as density increases,
Freezing
The process through which water changes from the
liquid to solid phase.
(c) Gaseous state
(Water vapor)
Melting
80 cal latent heat absorbed
Freezing
80 cal latent heat released
(a) Solid state
(Ice)
(b) Liquid state
(Water)
Figure 7.2 Physical states of water.
Arrows indicate the pathways that water follows in changing between various states, when latent
heat is either absorbed or released. Arrow colors reflect the net cooling or warming effect due to the transfer of sensible heat to latent heat
or latent heat to sensible heat, respectively. (a) Water exists as ice at temperatures below 0°C (32°F) under normal air pressure because the
molecules slow down enough to lock into hexagonal crystals. (b) At temperatures between 0°C (32°F) and 100°C (212°F), water molecules
move freely, but slowly enough to remain attached to one another in the liquid phase. (c) When temperatures exceed 100°C (212°F), all
water molecules move fast enough to become completely detached from one another in the vapor phase. (Note that clouds actually contain
liquid water as well as water vapor; we see them because the liquid water reflects light.) The number of calories shown as absorbed or
released in any given phase change assumes 1 g of water.
