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Encyclopedia > Vapor pressure

Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. All solids and liquids have a tendency to evaporate to a gaseous form, and all gases have a tendency to condense back. At any given temperature, for a particular substance, there is a partial pressure at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. This is the vapor pressure of that substance at that temperature. The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia. ... Vapor (US English) or vapour (British English) is the gaseous state of matter. ... In thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, and chemical equilibrium. ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... For other uses, see Solid (disambiguation). ... A liquid will usually assume the shape of its container A liquid is one of the main states of matter. ... Evaporation is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. ... This article does not cite any references or sources. ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ...


In meteorology, the term vapor pressure is used to mean the partial pressure of water vapor in the atmosphere, even if it is not equilibrium,[1] and the equilibrium vapor pressure is specified as such. Meteorologists also use the term saturation vapor pressure to refer to the equilibrium vapor pressure of water or brine above a flat surface, to distinguish it from equilibrium vapor pressure which takes into account the shape and size of water droplets and particulates in the atmosphere.[2] This article is about our first definition of vapor pressure, or what meteorologists would call equilibrium vapor pressure. // Meteorology (from Greek: μετέωρον, meteoron, high in the sky; and λόγος, logos, knowledge) is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting. ... In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... It has been suggested that multiple sections of steam be merged into this article or section. ... The saturation vapor pressure is the vapor pressure of water when air is saturated with water (having the maximum amount of water vapor that air can hold for a given temperature and pressure). ... For the sports equipment manufacturer, see Brine, Corp. ... The saturation vapor pressure is the vapor pressure of water when air is saturated with water (having the maximum amount of water vapor that air can hold for a given temperature and pressure). ...


Equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of molecules and atoms to escape from a liquid or a solid. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The higher the vapor pressure of a material at a given temperature, the lower the boiling point. 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... For other uses, see Atom (disambiguation). ... The ability of a liquid to evaporate quickly and at relatively low temperatures. ... Italic text This article is about the boiling point of liquids. ...


The vapor pressure of any substance increases non-linearly with temperature according to the Clausius-Clapeyron relation. The boiling point of a liquid is the temperature where the vapor pressure equals the ambient atmospheric pressure. At the boiling temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form bubbles inside the bulk of the substance. The Clausius-Clapeyron relation, in thermodynamics, is a way of characterizing the phase transition between two states of matter, such as solid and liquid. ...

Contents

Units of vapor pressure

The most common unit for vapor pressure is the torr. 1 torr = 1 mm Hg (one millimeter of mercury). The international unit for pressure is: 1 pascal = a force of 1 newton per square meter = 10 dyn/cm² = 0.01 mbar= 0.0075 mmHg = 0.00000969 atm= 0.0003545414 psi . The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... The torr is a unit of pressure. ... The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ... The newton (symbol: N) is the SI derived unit of force, named after Sir Isaac Newton in recognition of his work on classical mechanics. ... In physics, the dyne is a unit of force specified in the centimetre-gram-second (cgs) system of units, symbol dyn. One dyne is equal to exactly 10-5 newtons. ... A millibar (mbar, also mb) is 1/1000th of a bar, a unit for measurement of pressure. ... Standard atmosphere (symbol: atm) is a unit of pressure. ... A pressure gauge reading in PSI (red scale) and kPa (black scale) The pound-force per square inch (symbol: lbf/in²) is a non-SI unit of pressure based on avoirdupois units. ...


Equilibrium vapor pressure of solids

Equilibrium vapor pressure can be defined as the pressure reached when a condensed phase is in equilibrium with its own vapor. In the case of an equilibrium solid, such as a crystal, this can be defined as the pressure when the rate of sublimation of a solid matches the rate of deposition of its vapor phase. For most solids this pressure is very low, but some notable exceptions are naphthalene, dry ice (the vapor pressure of dry ice is 5.73 MPa (831 psi, 56.5 atm) at 20 degrees Celsius, meaning it will cause most non-ventilated containers to explode if sealed inside), and ice. Ice will still continue to disappear even though the ambient temperature is below the freezing point of water. All solid materials have a vapor pressure. However, due to their often extremely low values, measurement can be rather difficult. Typical techniques include the use of thermogravimetry and gas transpiration. For other uses, see Crystal (disambiguation). ... Sublimation of an element or substance is a conversion between the solid and the gas phases with no intermediate liquid stage. ... Naphthalene (not to be confused with naphtha) (also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or naphthene), is a crystalline, aromatic, white, solid hydrocarbon, best known as the primary ingredient of mothballs. ... Dry ice is a genericized trademark for solid (frozen) carbon dioxide. ... Thermogravimetry (also knows by acronym TG and obsolete names thermo-gravimetry, thermogravimmetry, thermography) is a branch of physical chemistry, materials research, and thermal analysis. ...


Relation between solid and liquid vapor pressures

It may be noted that the vapor pressure of a substance in liquid form is usually 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. This temperature is referred to as the melting point. In physics, melting is the process of heating a solid substance to a point (called the melting point) where it turns into a liquid. ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... The melting point of a crystalline solid is the temperature at which it changes state from solid to liquid. ...


Water vapor pressure

Main article: Vapor pressure of water
Graph of water vapor pressure versus temperature. Note that at the boiling point of 100°C, the vapor pressure equals the standard atmospheric pressure of 760 Torr.
Graph of water vapor pressure versus temperature. Note that at the boiling point of 100°C, the vapor pressure equals the standard atmospheric pressure of 760 Torr.

Water, like all liquids, starts to boil when its vapor pressure reaches its surrounding pressure. At higher elevations the atmospheric pressure is lower and water will boil at a lower temperature. The boiling temperature of water for pressures around 100 kPa can be approximated by Vapour pressure of water can be used in many experiments, particularly experiments relating to gases. ... Image File history File links Water_vapor_pressure_graph. ... Image File history File links Water_vapor_pressure_graph. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ...


T_v = 100 + 0.0002772 cdot (p - 101000) - 1.24 cdot 10^{-9} cdot (p - 101000)^2


where the temperature Tv is in degrees Celsius and the pressure p is in pascals. One gets the vapor pressure by solving this equation for p. Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). ... The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ...


In meteorology, the international standard for the vapour pressure of water over a flat surface is given by the Goff-Gratch equation. Vapour pressure of water can be used in many experiments, particularly experiments relating to gases. ... The Goff-Gratch Equation is the international standard for the computation of saturation vapor pressure. ...


Partial pressures

Raoult's law gives an approximation to the vapor pressure of mixtures of liquids. It states that the activity (pressure or fugacity) of a single-phase mixture is equal to the mole-fraction-weighted sum of the components' vapor pressures: In chemistry, Raoults law states that the vapor pressure of mixed liquids is dependent on the vapor pressures of the individual liquids and the molar vulgar fraction of each present in solution. ... Fugacity is a measure of the tendency of a substance to prefer one phase (liquid, solid, gas) over another. ...

ptot = piχi
i

where p is vapor pressure, i is a component index, and χ is a mole fraction. The term piχi is the vapor pressure of component i in the mixture. Raoult's Law is applicable only to non-electrolytes (uncharged species); it is most appropriate for non-polar molecules with only weak intermolecular attractions (such as London forces). In mathematics, an index is a superscript or subscript to a symbol. ... The mole fraction is one way of expressing the relative concentration of a given species. ... The title given to this article is incorrect due to technical limitations. ...


Systems that have vapor pressures higher than indicated by the above formula are said to have positive deviations. Such a deviation suggests weaker intermolecular attraction than in the pure components, so that the molecules can be thought of as being "held in" the liquid phase less strongly than in the pure liquid. An example is the azeotrope of approximately 95% ethanol and water. Because the azeotrope's vapor pressure is higher than predicted by Raoult's law, it boils at a temperature below that of either pure component. This article needs more context around or a better explanation of technical details to make it more accessible to general readers and technical readers outside the specialty, without removing technical details. ...


There are also systems with negative deviations that have vapor pressures that are lower than expected. Such a deviation is evidence for stronger intermolecular attraction between the constituents of the mixture than exists in the pure components. Thus, the molecules are "held in" the liquid more strongly when a second molecule is present. An example is a mixture of trichloromethane (chloroform) and 2-propanone (acetone), which boils above the boiling point of either pure component.


Examples of vapor pressures

gas vapor pressure
(bar)
vapor pressure
(mmHg)
Temperature
Helium 1 750 @ -269.15 °C
Propane 22 16500 @ 55 °C
Butane 2.2 1650 @ 20 °C
Carbonyl sulfide 12.55 9412 @ 25 °C
Acetaldehyde 0.987 740 @ 20 °C
Freon 113 0.379 284 @ 20 °C
Methyl isobutyl ketone 0.02648 19.86 @ 25 °C
Tungsten 0.001 0.75 @ 3203 °C

General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Standard atomic weight 4. ... Propane is a three-carbon alkane, normally a gas, but compressible to a liquid that is transportable. ... Butane, also called n-butane, is the unbranched alkane with four carbon atoms, CH3CH2CH2CH3. ... Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references Carbonyl sulfide is a colourless gas at room temperature with an unpleasant odor. ... R-phrases , , S-phrases , , , Flash point −39 °C Autoignition temperature 185 °C RTECS number AB1925000 Supplementary data page Structure and properties n, εr, etc. ... Freon is a trade name for a group of chlorofluorocarbons used primarily as a refrigerant. ... A pollutant that the government wants added to ethanol alcohol to prevent it from being used as a beverage, but only as a vehicle fuel instead. ... For other uses, see Tungsten (disambiguation). ...

See also

Absolute humidity is the mass of water vapor in a given volume of air or gas, expressed by weight and usually measured in grams per cubic meter, though grains per cubic foot has also been used in the United States. ... The Clausius-Clapeyron relation, in thermodynamics, is a way of characterizing the phase transition between two states of matter, such as solid and liquid. ... This does not adequately cite its references or sources. ... Vapor pressure of water can be used in many experiments, particularly experiments relating to gases. ...

External links


  Results from FactBites:
 
Vapor Pressure (610 words)
The temperature at which the vapor pressure is equal to the atmospheric pressure is called the boiling point.
The pressure of this equilibrium is called the saturation vapor pressure.
The boiling point is defined as the temperature at which the saturated vapor pressure of a liquid is equal to the surrounding atmospheric pressure.
Vapor pressure - Wikipedia, the free encyclopedia (593 words)
Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases.
This is the equilibrium vapor pressure or saturation vapor pressure of that substance at that temperature.
It may be noted that the vapor pressure of a substance in liquid form is usually different from the vapor pressure of the same substance in solid form.
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