# Vapor Pressure

views updated Jun 11 2018

# Vapor Pressure

Vapor pressure is the pressure that occurs because of the formation of vapor, or gas, from a liquid or solid. Within chemistry, this pressure generally indicates the rate at which particles (atoms or molecules) exit a substance to form vapor with respect to the rate that particles enter the same substance from the vapor. This pressure rate within chemistry is called equilibrium vapor pressure.

Scientists generally measure vapor pressure in the following units: physical atmosphere (atm) and millimeters of mercury (mm Hg or torr). For instance, the vapor pressure of water at 68° F (20° C) is 0.023 atm (17.5 mm Hg). Waters vapor pressure is very important to life forms on Earth because its value is sufficiently high to let the process of evaporation to occur, but sufficiently low to also allow water to exist in liquid and solid forms.

Scientists generally measure vapor pressure in the following units: physical atmosphere (atm) and millimeters of mercury (mm Hg or torr). For instance, the vapor pressure of water at 68° F (20° C) is 0.023 atm (17.5 mm Hg). Waters vapor pressure is very important to life forms on Earth because its value is sufficiently high to let the process of evaporation to occur, but sufficiently low to also allow water to exist in liquid and solid forms.

Vaporization of a liquid or sublimation of a solid may occur over a wide range of temperature and pressure. Wet clothes will dry, and a pan of water will slowly evaporate to dryness. Below the freezing point, frozen clothes will dry and a pan of ice cubes will slowly evaporate without first melting. Under virtually all conditions, some of the molecules near the surface of a liquid or solid attain enough energy to pull away from the attraction of their neighbors and escape into the gas or vapor phase.

Table 1. Equilibrium vapor pressure of water. (Thomson Gale.)
Equilibrium vapor pressure of water
Temperature °CWater vapor pressure mm HgTemperature °CWater vapor pressure mm Hg
04.660149
109.270237
2017.580355
3031.890526
4055100760
5093

Similarly, molecules in the gas phase occasionally strike the surface; and they are then captured by the attraction of molecules in the liquid or solid phase. When the liquid or solid is not confined, gas phase molecules will usually move away from the liquid or solid, and a few others will reunite with the liquid or solid phase. In this case, the liquid or solid will eventually evaporate or sublimate completely. If, however, a liquid or solid is confined in a closed container, a point is reached when the number of molecules returning to the liquid or solid phase from the vapor is equal to the number escaping. As mentioned earlier, this circumstance is called equilibrium.

All liquids and solids have vapor pressure at all temperatures except at absolute zero, -459° F (-273° C). The pressure of the vapor that is formed above its liquid or solid is called the vapor pressure. If a substance is in an enclosed place, the two-phase system will arrive to an equilibrium state. This equilibrium state is a dynamic, balanced condition with no change of either phase. The pressure of the vapor measured at equilibrium state is the equilibrium vapor pressure. This pressure is a fraction of the total pressure, which is equal to 760 mm Hg at sea level.

For a given substance, vapor pressure is constant under isothermal and isobarometric conditions, but its value depends on the temperature, pressure, and on the nature of the substance. As temperature increases so does the vapor pressure. At a constant temperature and pressure existing inter-molecular forces of the substance are the determining factors of the vapor pressure. The molecules of polar liquids and solids are held together with relatively large inter-molecular forces (e.g., dipole-dipole forces and hydrogen bounding). Polar compounds such as water, acetic acid, and ethyl alcohol have low vapor pressure at a given temperature. Non-polar liquids like ether, hexane, and benzene or solids like naphthalene have relatively small intermolecular forces (no hydrogen bounding or dipole-dipole forces). These substances have relatively high vapor pressure and are known as volatile substances. However, it should be noted that substances of high molecular weight evaporate more slowly than similar substances of low molecular weight.

The atmosphere has considerable water vapor in it; noted in the weather report as relative humidity. This relative humidity can be calculated by the equation below.

The water vapor present in the air is temperature, geography, and weather dependent. Many living systems, including humans, are affected by humidity. On a cold, wintry day, the air is dry due to the very low water vapor pressure (as low as 4 mm Hg) in the air. On a hot, humid summer day, the humidity can be above 40 mm Hg, in which is close to the equilibrium vapor pressure resulting in about 90% relative humidity. People use devices like humidifiers and dehumidifiers to compensate for such extreme conditions and keep the relative humidity level around 55 to 60%.

The equilibrium vapor pressure of water at different temperatures is given in Table 1.

Jeanette Vass

# Vapor Pressure

views updated Jun 27 2018

# Vapor pressure

## Definition

Vapor pressure is a force exerted by the gaseous phase of a two phase—gas/liquid or gas/solid system.

All liquids and solids have vapor pressure at all temperatures except at absolute zero , -459°F (-273°C). The pressure of the vapor that is formed above its liquid or solid is called the vapor pressure. If a substance is in an enclosed place the two phase system will arrive to an equilibrium state. This equilibrium state is a dynamic, balanced condition with no change of either phase. The pressure of the vapor measured at equilibrium state is the equilibrium vapor pressure. This pressure is a fraction of the total pressure, which is equal to 760 mm Hg at sea level . For a given substance, vapor pressure is constant under isothermal and isobarometric conditions, but its value depends on the temperature , pressure, and on the nature of the substance. As temperature increases so does the vapor pressure. At a constant temperature and pressure existing inter-molecular forces of the substance are the determining factors of the vapor pressure. The molecules of polar liquids and solids are held together with relatively large inter-molecular forces (e.g., dipole-dipole forces and hydrogen bounding). Polar compounds such as water , acetic acid, and ethyl alcohol have low vapor pressure at a given temperature. Non-polar liquids like ether , hexane, and benzene or solids like naphthalene have relatively small intermolecular forces (no hydrogen

 TEMPERATURE °C WATER VAPOR PRESSURE mm Hg TEMPERATURE °C WATER VAPOR PRESSURE mm Hg 0 4.6 60 149 10 9.2 70 237 20 17.5 80 355 30 31.8 90 526 40 55 100 760 50 93

bounding or dipole-dipole forces). These substances have relatively high vapor pressure and are known as volatile substances. However, it should be noted that substances of high molecular weight evaporate more slowly than similar substances of low molecular weight.

The atmosphere has considerable water vapor in it; noted in the weather report as relative humidity . This relative humidity can be calculated by the equation below.

The water vapor present in the air is temperature, geography, and weather dependent. Many living systems, including humans, are effected by humidity. On a cold, wintry day the air is dry due to the very low water vapor pressure (as low as 4 mm Hg) in the air. On a hot, humid summer day the humidity can be above 40 mm Hg, in which is close to the equilibrium vapor pressure resulting ~ 90% relative humidity. People use devices like humidifiers and dehumidifiers to compensate for such extreme conditions and keep the relative humidity level around 55-60%.

The equilibrium vapor pressure of water at different temperatures is given in Table 1.

Jeanette Vass

# vapour pressure

views updated May 08 2018

vapour pressure The pressure exerted by molecules of a substance in the vapour state, at equilibrium with molecules of the same substance in the liquid state, within a closed container. The magnitude of the vapour pressure exerted depends on the temperature and the identity of the liquid; it does not depend on the amount of liquid in the container. The saturated vapour pressure of water at 0°C is 610 Nm−2, rising to 2340 N m−2 at 20°C and 7380 N m−2 at 40°C. See also PARTIAL PRESSURE.

# vapour pressure

views updated May 18 2018

vapour pressure Pressure exerted by a vapour when it evaporates from a liquid or solid. When as many molecules leave to form vapour as return (in an enclosed space), this equilibrium is called a saturated vapour pressure. When a solid is dissolved in a liquid, the vapour pressure of the liquid is reduced by an amount proportional to the solid's relative molecular mass.