Also found in: Dictionary, Thesaurus, Medical, Acronyms, Wikipedia.
in mechanics, ratio of the force acting on a surface to the area of the surface; it is thus distinct from the total force acting on a surface. A force can be applied to and sustained by a single point on a solid.
..... Click the link for more information. 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 equilibriumchemical equilibrium,
state of balance in which two opposing reversible chemical reactions proceed at constant equal rates with no net change in the system. For example, when hydrogen gas, H2, and iodine gas, I2
..... Click the link for more information. ). 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 principleLe Châtelier's principle,
chemical principle that states that if a system in equilibrium is disturbed by changes in determining factors, such as temperature, pressure, and concentration of components, the system will tend to shift its equilibrium position so as to
..... Click the link for more information. 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 lawRaoult's law
[for F. M. Raoult, a French physicist and chemist] states that the addition of solute to a liquid lessens the tendency for the liquid to become a solid or a gas, i.e., reduces the freezing point and the vapor pressure (see solution).
..... Click the link for more information. , the vapor pressure of a pure liquid or solid is lowered by the addition of a solute.
in meteorology, the partial pressure of water vapor in the atmosphere, expressed in millimeters of Hg or in millibars (1 millibar = 102 newtons per m2). Vapor pressure depends on the amount of water vapor per unit volume; it constitutes one of the characteristics of atmospheric humidity. Vapor pressure readings at the earth’s surface range from close to zero (in Antarctica, in the Yakut ASSR in winter, and sometimes in deserts) to 30–35 millibars (near the equator). Vapor pressure decreases rapidly with increased altitude—by 50 percent within the lower 1.5 km of the atmosphere and almost to zero at the upper boundary of the troposphere.