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vapor pressure

vapor pressure,pressure exerted by a vapor that is in equilibrium with its liquid. A liquid standing in a sealed beaker is actually a dynamic system: some molecules of the liquid are evaporating to form vapor and some molecules of vapor are condensing to form liquid. At equilibrium the rates of the two processes are equal and the system appears to be stationary (see chemical equilibrium). The vapor, like any gas, exerts a pressure, and this pressure at equilibrium is called the vapor pressure. Vapor pressure depends on various factors, the most important of which is the nature of the liquid. If the molecules of liquid bind to each other very strongly, there will be less tendency for the molecules to escape as gas and a consequent lower vapor pressure; for example, polar molecules that can form hydrogen bonds between themselves, e.g., water molecules and the alcohols, have relatively low vapor pressures. If there is only weak interaction between the liquid molecules, there will be a greater tendency for the molecules to evaporate and a higher vapor pressure. Temperature also affects the vapor pressure. If the system in equilibrium is perturbed by raising the temperature, then according to Le Châtelier's principle the system should react to relieve this stress; as the temperature is increased, the evaporation process, which absorbs heat, is speeded up to a greater degree than the condensation process, which gives off heat, so that the vapor pressure is higher when equilibrium is restored at the new temperature. If the temperature is increased enough to raise the vapor pressure until it equals atmospheric pressure, the liquid will boil. If the external pressure is reduced, as in a vacuum system, then the liquid will boil much more readily than under atmospheric pressure. This fact is used in the vacuum distillation process to obtain relatively pure samples of liquids with high boiling points. Some solids, e.g., iodine and carbon dioxide, are capable of subliming (going directly from a solid to a gas) at atmospheric pressure and room temperature; thus, such solids also have significant vapor pressures under these conditions. Another factor affecting vapor pressure is the presence of dissolved substances in the liquid or solid; according to Raoult's law, the vapor pressure of a pure liquid or solid is lowered by the addition of a solute.

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vapour pressure

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.

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vapour pressure

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.

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Vapor Pressure

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 °C Water vapor pressure mm Hg Temperature °C Water 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

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Vapor Pressure

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

TABLE 1. EQUILIBRIUM VAPOR PRESSURE OF WATER
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

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