The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor. At any given temperature, for a particular substance, there is a pressure at which the vapor of that substance is in equilibrium with its liquid or solid forms. This is the equilibrium vapor pressure or saturation vapor pressure of that substance at that temperature. The term ''vapor pressure'' is often understood to mean saturated vapor pressure. A substance with a high vapor pressure at normal temperatures is often referred to as ''volatile''. ==Liquids== When the partial pressure of any liquid ''equals'' its vapor pressure, the liquid is partially vaporized: liquid and vapor are in equilibrium. Given a constant temperature, if the pressure is reduced, the equilibrium is changed in favour of the substance's gas phase: The liquid eventually gets totally vaporized. If pressure is increased, the opposite occurs: Eventually, all vapor will condensation to liquid. With constant pressure but variable temperature, even lower temperatures will cause all vapor to condense to liquid, while a continual increase in temperature will cause the liquid to wholly evaporation (turn to vapor). At any given pressure, the boiling point of a substance is the temperature at which the vapor pressure of the substance ''in liquid form'' equals the total ambient pressure. The processes of condensation and evaporation can be delayed, which is referred to as supersaturation and superheating, respectively. ==Solids== When the ambient pressure equals the vapor pressure of any solid, the solid and vapor are in equilibrium. Below that temperature, vapor will condense to solid; above that temperature, solid will sublime (turn to vapor). Thus, at any given pressure, the sublimation point of a substance is the temperature at which the vapor pressure of the substance ''in solid form'' equals the ambient pressure. ==Relation== It may be noted that the vapor pressure of a substance in liquid form may be (and, in general, usually ''is'') different from the vapor pressure of the same substance in solid form. If the temperature is such that the vapor pressure of the liquid is higher than that of the solid, liquid will vaporize but vapor will condense to a solid, i.e. the liquid is freezing. If the temperature is such that the vapor pressure of the liquid is lower than that of the solid, solid will vaporize but vapor will condense to a liquid, i.e. the solid is melting. At the temperature that equalizes the two vapor pressures, an equilibrium exists between solid and liquid phases of matter. This temperature is referred to as the melting point. ==Water== The boiling temperature of water for pressures around 100 Pascal can be approximated by where the temperature is in degrees Celsius and the pressure ''p'' is in pascals. One gets the vapor pressure by solving this equation for ''p''. Raoult's law approximately governs the vapor pressure of mixtures of liquids. ==See also== * Relative humidity * Absolute humidity. Chemical properties
I think the last mentioned temperature is not the melting point but the triple point. Is that correct? :Actually, no. When the vapor pressures of liquid and solid are equal only if the two phases (liquid and solid) are in equilibrium. If these pressures are also equal to the total pressure, then the gas phase is in equilibuim, too ( three phase -> triple point) -- User:Olof And another question: to which substances does this discussion apply? Wood for instance doesn't have a vapor pressure, does it? : Sure it does. This is why I put wood in the garage for a year before putting it in the fireplace: some of its components ( water ) sublimate. However, wood is a multicomponent material, so it is better to think of vapor pressures of each of its components. -- User:Olof And yet another one: what is the vapor pressure of solid water at 50 degrees Celcius? user:AxelBoldt : Higher than the vapor pressure of liquid water at 50 degrees Celsius. The fact that we know that it is higher doesn't imply that we are able to actually measure the value, although it could be estimated by extrapolating data from the temperature range where water is stable. :I've got to admit, though, that this page is quite confusing, and could use a better explanation of all the concepts. -- User:Olof == comparing and contrasting the dependence of the temperature....... == In a simple distillation, am I correct in assuming that the temperature of the liquid and vapor are equal? I am trying to compare and contrast the dependence of temperature on the distillate volume of cyclohexane and toluene. I find it easy to compare the temperature with the mixture, but would like to find some contrasting factors. Email is charlespr01@hotmail.com Chuck
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Vapor_pressures_of_the_elements_(data_page)
Vapor_pressures_of_the_elements_(data_page)
Vapor pressure
The vapor pressure is the pressure (if the vapor is mixed with other gases, the partial pressure) of a vapor. At any given temperature, for a particular substance, there is a pressure at which the vapor of that substance is in equilibrium with its liquid or solid forms. This is the equilibrium vapor pressure or saturation vapor pressure of that substance at that temperature. The term ''vapor pressure'' is often understood to mean saturated vapor pressure. A substance with a high vapor pressure at normal temperatures is often referred to as ''volatile''. ==Liquids== When the partial pressure of any liquid ''equals'' its vapor pressure, the liquid is partially vaporized: liquid and vapor are in equilibrium. Given a constant temperature, if the pressure is reduced, the equilibrium is changed in favour of the substance's gas phase: The liquid eventually gets totally vaporized. If pressure is increased, the opposite occurs: Eventually, all vapor will condensation to liquid. With constant pressure but variable temperature, even lower temperatures will cause all vapor to condense to liquid, while a continual increase in temperature will cause the liquid to wholly evaporation (turn to vapor). At any given pressure, the boiling point of a substance is the temperature at which the vapor pressure of the substance ''in liquid form'' equals the total ambient pressure. The processes of condensation and evaporation can be delayed, which is referred to as supersaturation and superheating, respectively. ==Solids== When the ambient pressure equals the vapor pressure of any solid, the solid and vapor are in equilibrium. Below that temperature, vapor will condense to solid; above that temperature, solid will sublime (turn to vapor). Thus, at any given pressure, the sublimation point of a substance is the temperature at which the vapor pressure of the substance ''in solid form'' equals the ambient pressure. ==Relation== It may be noted that the vapor pressure of a substance in liquid form may be (and, in general, usually ''is'') different from the vapor pressure of the same substance in solid form. If the temperature is such that the vapor pressure of the liquid is higher than that of the solid, liquid will vaporize but vapor will condense to a solid, i.e. the liquid is freezing. If the temperature is such that the vapor pressure of the liquid is lower than that of the solid, solid will vaporize but vapor will condense to a liquid, i.e. the solid is melting. At the temperature that equalizes the two vapor pressures, an equilibrium exists between solid and liquid phases of matter. This temperature is referred to as the melting point. ==Water== The boiling temperature of water for pressures around 100 Pascal can be approximated by where the temperature is in degrees Celsius and the pressure ''p'' is in pascals. One gets the vapor pressure by solving this equation for ''p''. Raoult's law approximately governs the vapor pressure of mixtures of liquids. ==See also== * Relative humidity * Absolute humidity. Chemical properties
Vapor pressure
I think the last mentioned temperature is not the melting point but the triple point. Is that correct? :Actually, no. When the vapor pressures of liquid and solid are equal only if the two phases (liquid and solid) are in equilibrium. If these pressures are also equal to the total pressure, then the gas phase is in equilibuim, too ( three phase -> triple point) -- User:Olof And another question: to which substances does this discussion apply? Wood for instance doesn't have a vapor pressure, does it? : Sure it does. This is why I put wood in the garage for a year before putting it in the fireplace: some of its components ( water ) sublimate. However, wood is a multicomponent material, so it is better to think of vapor pressures of each of its components. -- User:Olof And yet another one: what is the vapor pressure of solid water at 50 degrees Celcius? user:AxelBoldt : Higher than the vapor pressure of liquid water at 50 degrees Celsius. The fact that we know that it is higher doesn't imply that we are able to actually measure the value, although it could be estimated by extrapolating data from the temperature range where water is stable. :I've got to admit, though, that this page is quite confusing, and could use a better explanation of all the concepts. -- User:Olof == comparing and contrasting the dependence of the temperature....... == In a simple distillation, am I correct in assuming that the temperature of the liquid and vapor are equal? I am trying to compare and contrast the dependence of temperature on the distillate volume of cyclohexane and toluene. I find it easy to compare the temperature with the mixture, but would like to find some contrasting factors. Email is charlespr01@hotmail.com Chuck
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Words begining with Vapor_pressure:
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Vapor_pressures_of_the_elements_(data_page)
Vapor_pressures_of_the_elements_(data_page)
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