Page 126 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch04_085-114.indd Page 103 9/1/10 9:38 PM user-f465
tiL12214_ch04_085-114.indd Page 103 9/1/10 9:38 PM user-f465 /Users/user-f465/Desktop

25 25
10
5 10
5 15
10 10 5 5
5
5 5
15 5
10 10

115 65
Average = = 11.5 Average = = 8.1
10 8
FIGURE 4.21 Temperature is associated with the average
energy of the molecules of a substance. These numbered circles
represent arbitrary levels of molecular kinetic energy that, in turn,
represent temperature. The two molecules with the higher kinetic
energy values [25 in (A)] escape, which lowers the average values
from 11.5 to 8.1 (B). Thus, evaporation of water molecules with
more kinetic energy contributes to the cooling effect of evaporation
in addition to the absorption of latent heat.

the average kinetic energy of the water molecules. Th e word
average implies that some of the molecules have a greater
energy and others have less (refer to Figure 4.4). If a mol-
ecule of water that has an exceptionally high energy is near
the surface and is headed in the right direction, it may over-
come the attractive forces of the other water molecules and
escape the liquid to become a gas. This is the process of evapo-

ration. Evaporation reduces a volume of liquid water as water
FIGURE 4.22 The inside of this closed bottle is isolated from
molecules leave the liquid state to become water vapor in the the environment, so the space above the liquid becomes saturated.
atmosphere (Figure 4.21). While it is saturated, the evaporation rate equals the condensation
Water molecules that evaporate move about in all direc- rate. When the bottle is cooled, condensation exceeds evaporation
tions, and some will return, striking the liquid surface. Th e same and droplets of liquid form on the inside surfaces.
forces that they escaped from earlier capture the molecules,
returning them to the liquid state. This is called the process of

condensation. Condensation is the opposite of evaporation. In
evaporation, more molecules are leaving the liquid state than There are four ways to increase the rate of evaporation.

are returning. In condensation, more molecules are returning to (1) An increase in the temperature of the liquid will increase
the liquid state than are leaving. This is a dynamic, ongoing pro- the average kinetic energy of the molecules and thus increase

cess with molecules leaving and returning continuously. Th e net the number of high-energy molecules able to escape from the
number leaving or returning determines whether evaporation liquid state. (2) Increasing the surface area of the liquid will
or condensation is taking place (Figure 4.22). also increase the like lihood of molecular escape to the air.

When the condensation rate equals the evaporation rate, This is why you spread out wet clothing to dry or spread out
the air above the liquid is said to be saturated. The air immedi- a puddle you want to evaporate. (3) Removal of water vapor

ately next to a surface may be saturated, but the condensation from near the surface of the liquid will prevent the return of
of water molecules is easily moved away with air movement. the vapor molecules to the liquid state and thus increase the
There is no net energy flow when the air is saturated, since net rate of evaporation. This is why things dry more rapidly

the heat carried away by evaporation is returned by con- on a windy day. (4) Reducing the atmospheric pressure will

densation. This is why you fan your face when you are hot. increase the rate of evaporation. The atmospheric pressure
The moving air from the fanning action pushes away water and the intermolecular forces tend to hold water molecules in

molecules from the air near your skin, preventing the adja- the liquid state. Thus, reducing the atmospheric pressure will
cent air from becoming saturated, thus increasing the rate of reduce one of the forces holding molecules in a liquid state.

evaporation. Think about this process the next time you see Perhaps you have noticed that wet items dry more quickly at
someone fanning his or her face. higher elevations, where the atmospheric pressure is less.
4-19 CHAPTER 4 Heat and Temperature 103