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Vapor Pressure


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

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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