Vapor pressure pure substances

Consider a single, pure fluid at constant temperature, in a cylinder fitted with a friotionless piston. If a pressure is applied on the piston which is greater than the vapor pressure of the liquid, the system will consist entirely of liquid when equilibrium is reached. No vapor will be present since at pressures greater than the vapor pressure it condenses into liquid. If, on the other hand, the pressure applied on the piston is less than the vapor pressure of the liquid only vapor will be present at equilibrium. If both liquid and vapor are present in equilibrium with one another, the pressure must be exactly equal to the vapor pressure. Pure substances behave in this manner and liquid and vapor can coexist at a given temperature only at a pressure equal to the vapor pressure. The relative amounts of liquid and vapor that coexist is determined by the volume of the system, and can vary an3nvliere from an infinitesimal amount of liquid to an infinitesimal amount of vapor. 

In about 1886, Raoult discovered that substances have lower vapor pressures in solution than in pure form and that the freezing point of an aqueous solution decreases in proportion to the amount of a non-electrolytic substance dissolved. The ratio of the partial vapor pressure of substance i in solution to the vapor pressure of the pure substance (subscript 0 denotes the pure substance) is equal to the mole fraction x of i ... 

Water availability (AW) is defined as the water available for bacterial metabolism and is evaluated by measurement of the water vapor pressure at the surface of a product or a raw material. It can be defined as the ratio AW water vapor pressure over substance at r°/water vapor pressure over pure water at f. 

Boublik, T., V. Fried, and E. Hala "The Vapor Pressure of Pure Substances," Elsevier, Amsterdam, 1973. 

The calculation of vapor pressure of a pure substance consists of finding the pressure for which the fugacities of the liquid and vapor are equal. 

When the critical constants for a pure substance or the pseudocritical constants for a petroleum fraction are known, the vapor pressure for hydrocarbons and petroleum fractions can be calculated using the Lee and Kesler equations ... 

The short-cut technique frequentiy used to estimate the Henry s constant of a volatile substance ia water is to calculate the ratio of the pure compound s vapor pressure to its aqueous saturation limit (23) ... 

Additional compilations of vapor-pressure data include Boubhk, Fried, and Hala, The Vapor Pre.s.sure.s of Pure Substances, Elsevier, Amsterdam, 1984. See also Hirata, Ohe, and Nagahama, Computer Aided Data Book of Vapor-Liquid Equilibria, Kodansha/Elsevier, Tokyo, 1975 Weishaupt, Landolt-Bornstein New Series Group TV, vol. 3 Thermodynamic Equilibria of Boiling Mixtures, Springer-Verlag, Berhn, 1975 Wichterle, Linek, and Hala, Vapor-Liquid Equilibrium Data Bibliography, Elsevier, Amsterdam, 1973 suppl. 1, 1976 suppl. 2,1982. 

Finally, we may note that every solution exerts a vapor pressure less than that of the pure solvent at the same temperature. Corresponding to this vapor-pressure lowering is an equivalent boiling-point raising. Dissolved substances lower the vapor pressure of the solvent. Such reduction increases with the concentration of solute. Since a solution boils when its vapor pressure reaehes that of its surroundings, it must be heated to a temperature above the boiling point of the pure... 

Vapor—Liquid Systems. The vapor-liquid region of a pure substance is contained within the phase or saturation envelope on a P-V diagram (see Figure 2-80), A vapor, whether it exists alone or in a mixture of gases, is said to be saturated if its partial pressure (P.) equals its equilibrium vapor pressure (P, ) at the system temperature T. This temperature is called the saturation temperature or dew point T ... 

Clausius-Clapeyron Equation. This equation was originally derived to describe the vaporization process of a pure liquid, but it can be also applied to other two-phase transitions of a pure substance. The Clausius-Clapeyron equation relates the variation of vapor pressure (P ) with absolute temperature (T) to the molar latent heat of vaporization, i.e., the thermal energy required to vajxirize one mole of the pure liquid ... 

Molecular Weight Determination by Application of Raoult s Law. If a small amount (m in grams) of a nonvolatile, nonionized substance (solute, 2) is dissolved in m, grams of a volatile liquid (solvent, 1), it experiences a lowering of vapor pressure from the pure solvent value (P ) to the solution value (P) at the system temperature. This is a consequence of Raoult s law because the total vapor pressure of the dilute solution (x 1) is given by P = x P + x P = 1 -... 

It is impossible to have liquid carbon dioxide at temperatures above 31°C, no matter how much pressure is applied. Even at pressures as high as 1000 atm, carbon dioxide gas does not liquefy at 35 or 40°C. This behavior is typical of all substances. There is a temperature, called the critical temperature, above which the liquid phase of a pure substance cannot exist The pressure that must be applied to cause condensation at that temperature is called the critical pressure. Quite simply, the critical pressure is the vapor pressure of the liquid at the critical temperature. 

For a pure substance, having three phases in equilibrium results in a triple point that is invariant. When pure solid, liquid, and gaseous water are in equilibrium, the temperature is fixed at a value of 273.16 K, and the pressure of the gas is fixed at the vapor pressure value (0.6105 kPa). 

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