Vapor pressure evaporation rate

Substances with high vapor pressure evaporate rapidly. Those with low vapor pressure evaporate slowly. The impact of vapor pressure on the rate of evaporation makes vapor pressure a very important property in considering the tactical use and duration of effectiveness of chemical agents. A potential chemical agent is valuable for employment when it has a reasonable vapor pressure. One with exceptionally high vapor pressure is of limited use. It vaporizes and dissipates too quickly. Examples are arsine and carbon monoxide. On the other hand, mechanical or thermal means may effectively aerosolize and disseminate solid and liquid agents of very low vapor pressure. Vapor pressure and volatility are related. Translated into volatility, vapor pressure is most understandable and useful. 

Liquids with high saturation vapor pressures evaporate faster. As a result, the evaporation rate (mass/time) is expected to be a function of the saturation vapor pressure. In reality, for vaporization into stagnant air, the vaporization rate is proportional to the difference be-... 

Odor and appearance Odor threshold Specific gravity Vapor pressure Vapor density Evaporation rate Boiling point Freezing point pH... 

V apor pressure is an important property of liquids, and to a much lesser extent, of solids. If a liquid is allowed to evaporate in a confined space, tlie pressure of tlie vapor phase increases as tlie amount of vapor increases. If tliere is sufficient liquid present, tlie pressure in tlie vapor space eventually comes to equal exactly tlie pressure exerted by the liquid at its own surface. At tliis point, a dynamic equilibrium exists in wliich vaporization and condensation take place at equal rates and tlie pressure in tlie vapor space remains constant. The pressure exerted at equilibrium is called tlie vapor pressure of the liquid. Solids, like liquids, also exert a vapor pressure. Evaporation of solids (sublimation) is noticeable only for tlie few solids characterized by appreciable vapor pressures. 

As the brine becomes more concentrated, its vapor pressure and rate of evaporation become less. By the end of NaCl precipitation, the rate will have dropped nearly by... 

The coefficient of linear thermal expansion, the thermal conductivity, the specific heat, and the electrical resistivity as function of temperature are shown in Figs. 3.1-140-3.1-143. The vapor pressure and rate of evaporation are shown in Fig. 3.1-144. In the case of precipitation- and dispersion-strengthened molybdenum alloys, such as TZM, MHC, ML, MY, and K—Si—Mo,... 

A What are some of the similarities of the molecular-level descriptions of the viscosity surface tension, vapor pressure, and rate of evaporation of a liquid ... 

Dynamic equilibrium, liquid-vapor equilibrium, evaporation rate, condensation rate, temperature, vapor pressure... 

FIGURE 11.1 An example of a physical equilibrium first discussed in Chapter 9 (see also Figure 9.9) to define vapor pressure. The rate of evaporation of a liquid in a closed container is equal to the rate of condensation of the vapors. The quantity of the liquid or the vapor does not change even though both evaporation and condensation are occurring. 

VAPOR PRESSURE AND RATE OF EVAPORATION (TOTAL) (continued)... 

An interesting consequence of covering a surface with a film is that the rate of evaporation of the substrate is reduced. Most of these studies have been carried out with films spread on aqueous substrates in such cases the activity of the water is practically unaffected because of the low solubility of the film material, and it is only the rate of evaporation and not the equilibrium vapor pressure that is affected. Barnes [273] has reviewed the general subject. 

Not all molecules striking a surface necessarily condense, and Z in Eq. VII-2 gives an upper limit to the rate of condensation and hence to the rate of evaporation. Alternatively, actual measurement of the evaporation rate gives, through Eq. VII-2, an effective vapor pressure Pe that may be less than the actual vapor pressure P. The ratio Pe/P is called the vaporization coefficient a. As a perhaps extreme example, a is only 8.3 X 10" for (111) surfaces of arsenic [11]. 

Volatilization. The susceptibility of a herbicide to loss through volatilization has received much attention, due in part to the realization that herbicides in the vapor phase may be transported large distances from the point of application. Volatilization losses can be as high as 80—90% of the total applied herbicide within several days of application. The processes that control the amount of herbicide volatilized are the evaporation of the herbicide from the solution or soHd phase into the air, and dispersal and dilution of the resulting vapor into the atmosphere (250). These processes are influenced by many factors including herbicide application rate, wind velocity, temperature, soil moisture content, and the compound s sorption to soil organic and mineral surfaces. Properties of the herbicide that influence volatility include vapor pressure, water solubility, and chemical stmcture (251). 

Alloys. Alloys consist of two or mote elements of different vapor pressures and hence different evaporation rates. As a result, the vapor phase and therefore the deposit constantiy vary in compositions. This problem can be solved by multiple sources or a single rod- or wire-fed electron beam source fed with the alloy. These solutions apply equally to evaporation or ion-plating processes. 

The evaporation rate from the gel is dictated by the difference between the vapor pressure at the evaporating surface, P, and the vapor pressure of the ambient atmosphere, P. Evaporation continues as long as at a rate, E), of... 

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