40
Water Vapor and Other Gases
3.2 Water Vapor: Saturation Conditions
If a container of pure water is uncovered in an evacuated, closed space,
water will evaporate into the space above the liquid water. As water evaporates, the concentration of water molecules in the gas phase increases.
Finally, an equilibrium is established when the number of molecules escaping from the liquid water equals the number being recaptured by the
liquid. If the temperature of the liquid was increased, the random kinetic
energy of the molecules would increase, and more water would escape.
The equilibrium vapor pressure, established between liquid water and water vapor in a closed system is known as the saturation vapor pressure for
the particular temperature of the system. The saturation vapor pressure
is the highest pressure of water vapor that can exist in equilibrium with
a plane, free water surface at a given temperature. The saturation vapor
pressure is shown as a function of temperature in Fig. 3.1. Since the
influence from any other gases present in the space above the water is
negligible, the vapor pressure above the water surface is essentially the
same whether the closed space is initially evacuated or contains air or
other gases. The symbol e is used to represent the vapor pressure of water, and the saturation vapor pressure is denoted by e,(T), indicating that
the saturation vapor pressure is determined by temperature. Tables giving
saturation vapor pressure as a function of temperature can be found in
List (1971) and in Table A.3 of the Appendix of this book. The mole fraction, which we frequently use in future computations, depends on both
temperature and pressure. It is computed using Eq. (3.5), by dividing
e, (T) by the atmospheric pressure. The main variable determining atmoI
I
I
I
I
I
0
10
20
30
40
Temperature ( C )
FIGURE 3-1. Saturation vapor pressure of air as a function of temperature.
Water Vapor and Other Gases
3.2 Water Vapor: Saturation Conditions
If a container of pure water is uncovered in an evacuated, closed space,
water will evaporate into the space above the liquid water. As water evaporates, the concentration of water molecules in the gas phase increases.
Finally, an equilibrium is established when the number of molecules escaping from the liquid water equals the number being recaptured by the
liquid. If the temperature of the liquid was increased, the random kinetic
energy of the molecules would increase, and more water would escape.
The equilibrium vapor pressure, established between liquid water and water vapor in a closed system is known as the saturation vapor pressure for
the particular temperature of the system. The saturation vapor pressure
is the highest pressure of water vapor that can exist in equilibrium with
a plane, free water surface at a given temperature. The saturation vapor
pressure is shown as a function of temperature in Fig. 3.1. Since the
influence from any other gases present in the space above the water is
negligible, the vapor pressure above the water surface is essentially the
same whether the closed space is initially evacuated or contains air or
other gases. The symbol e is used to represent the vapor pressure of water, and the saturation vapor pressure is denoted by e,(T), indicating that
the saturation vapor pressure is determined by temperature. Tables giving
saturation vapor pressure as a function of temperature can be found in
List (1971) and in Table A.3 of the Appendix of this book. The mole fraction, which we frequently use in future computations, depends on both
temperature and pressure. It is computed using Eq. (3.5), by dividing
e, (T) by the atmospheric pressure. The main variable determining atmoI
I
I
I
I
I
0
10
20
30
40
Temperature ( C )
FIGURE 3-1. Saturation vapor pressure of air as a function of temperature.