Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure), is the pressure of a vapor in equilibrium with its non-vapor phases. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface A vapor or vapour (see Spelling differences) is a substance in the Gas phase at a Temperature lower than its Critical temperature In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and 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 All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to condense back into their original form (either liquid or solid). Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Evaporation is the process by which Molecules in a Liquid state (e This page is about the physical properties of gas as a state of matter Condensation is the change of the physical state of aggregation (or simply state of matter from gaseous phase into liquid phase At any given temperature, for a particular substance, there is a pressure at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface This is the vapor pressure of that substance at that temperature. The 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. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny A substance with a high vapor pressure at normal temperatures is often referred to as volatile. Volatility in the context of Chemistry, Physics and Thermodynamics is a measure of the tendency of a substance to Vaporize. The Kelvin equation shows how equilibrium vapor pressure depends on droplet size. Kelvin equation describes the change of vapour pressure over liquid curved with a radius r (for example in a Capillary or over a droplet

An example is water vapor when air is saturated with water vapor. General properties of water vapor Evaporation/sublimation Whenever a water molecule leaves a surface it is said to have evaporated In Chemistry, saturation has five different meanings In Physical chemistry, saturation is the point at which a Solution of a substance It is the vapor pressure usually found over a flat surface of liquid water, ^[1] and is a dynamic equilibrium where the rate of condensation of water equals the rate of evaporation of water. A dynamic equilibrium occurs when two opposing Processes proceed at the same rate Condensation is the change of the physical state of aggregation (or simply state of matter from gaseous phase into liquid phase Water is a common Chemical substance that is essential for the survival of all known forms of Life. Evaporation is the process by which Molecules in a Liquid state (e Water is a common Chemical substance that is essential for the survival of all known forms of Life. In general, the higher the temperature, the higher the vapor pressure. When air is at the saturation vapor pressure, it is said to be at the dew point. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five The dew point (sometimes spelled dewpoint) is the Temperature to which a given parcel of Air must be cooled at constant Barometric pressure, Thus, at saturation vapor pressure, air has a relative humidity of 100% and condensation occurs with any increase of water vapor content or a reduction in temperature. Relative humidity is a measurement of the amount of Water vapor that exists in a gaseous mixture of air and water Condensation is the change of the physical state of aggregation (or simply state of matter from gaseous phase into liquid phase Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature

The international standard for saturation vapor pressure over water is given by the Goff-Gratch equation. The Goff-Gratch equation is one (arguably the first reliable amongst many equations that have been proposed to estimate the saturation water vapor pressure at a given temperature Another more recent equation for water is the Arden Buck Equation. An equation is a mathematical statement, in symbols, that two things are exactly the same (or equivalent Water is a common Chemical substance that is essential for the survival of all known forms of Life. The Arden Buck Equation is an equation describing the Saturation vapor pressure of water at various temperatures

Assuming absolutely clean air, if water droplets have a high curvature, which is the case when they are smaller, they require relative humidities in excess of 100% (known as supersaturation) to be at an equilibrium vapor pressure. The term supersaturation refers to a Solution that contains more of the dissolved material than could be dissolved by the Solvent under normal circumstances As droplets approach approximately 20 micrometers, they can survive at 100% relative humidity. Year 20 was a Leap year starting on Monday (link will display the full calendar of the Julian calendar. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre As the droplet grows larger by collision and coalescence, it can survive longer because its curvature becomes smoother as the droplet grows. Coalescence is the process by which two or more droplets or particles merge during contact to form a single daughter droplet (or bubble Of course, in actual practice in the Earth's atmosphere, the ability of water to condense into droplets is generally affected by the presence of hygroscopic dust particles (Cloud Condensation nuclei). Hygroscopy is the ability of a substance to attract Water Molecules from the surrounding environment through either absorption or Adsorption Cloud condensation nuclei or CCN s (also known as cloud seeds) are small particles (typically 0 The relative humidity required for droplets to actually form can be significantly below the real saturation vapor pressure due to the solute effect. Finally, if the temperature becomes low enough in a cloud, as it does in nimbostratus and cumulonimbus clouds, microscopic ice crystals may also serve as condensation nuclei for the cloud in a process known as the Bergeron process. Nimbo is from the Latin word "nimbus" meaning rain A Nimbostratus is a Cloud of the class characterized by a formless layer that is almost uniformly dark gray Cumulonimbus (Cb is a type of Cloud that is tall dense and involved in Thunderstorms and other intense Weather. The Bergeron Findeisen Process (after Tor Bergeron and W Findeisen is the formation of precipitation in the cold clouds of the mid and upper latitudes by ice crystal

The vapor pressure of any substance increases non-linearly with temperature according to the Clausius-Clapeyron relation. The Clausius-Clapeyron relation, named after Rudolf Clausius and Émile Clapeyron, is a way of characterizing the Phase transition between two phases of The atmospheric pressure boiling point of a liquid (also known as the normal boiling point) is the temperature where the vapor pressure equals the ambient atmospheric pressure. The boiling point of a liquid is the temperature at which the Vapor pressure of the liquid equals the environmental pressure surrounding the liquid The boiling point of a liquid is the temperature at which the Vapor pressure of the liquid equals the environmental pressure surrounding the liquid With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher pressure, and therefore higher temperature, because the fluid pressure increases above the atmospheric pressure as the depth increases.

Relation between vapor pressures and normal boiling points of liquids

A typical vapor pressure chart for various liquids

The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point (i. e. , the boiling point at atmospheric pressure) of the liquid.

The vapor pressure chart to the right has graphs of the vapor pressures versus temperatures for a variety of liquids. ^[2] As can be seen in the chart, the liquids with the highest vapor pressures have the lowest normal boiling points.

For example, at any given temperature, propane has the highest vapor pressure of any of the liquids in the chart. Propane is a three- Carbon Alkane, normally a gas but compressible to a liquid that is transportable It also has the lowest normal boiling point(-43. 7 °C), which is where the vapor pressure curve of propane (the purple line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapor pressure. The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm

Although the relation between vapor pressure and temperature is non-linear, the chart uses a logarithmic vertical axis in order to obtain slightly curved lines so that one chart can graph many liquids.

Units of vapor pressure

The international SI unit for pressure is the pascal (Pa), equal to one newton per square meter (N·m^-2 or kg·m^-1·s^-2). The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical M^2 redirects here For other uses see M². CM2 redirects here The conversions to other pressure units are:

Example reading:  1 Pa = 1 N/m²  = 10⁻⁵ bar  = 10. This article deals with the unit of force For the unit of mass see Pound (mass. 197×10⁻⁶ at  = 9. 8692×10⁻⁶ atm, etc.
Note:  mmHg is an abbreviation for millimetres of mercury.

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. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating Sublimation of an element or compound is a transition from the Solid to Gas phase with no intermediate liquid stage For most solids this pressure is very low, but some notable exceptions are naphthalene, dry ice (the vapor pressure of dry ice is 5. Naphthalene, also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or antimite and not to be Dry ice is solid Carbon dioxide. It is commonly used as a versatile cooling agent 73 MPa (831 psi, 56. 5 atm) at 20 degrees Celsius, meaning it will cause most sealed containers to explode), and ice. 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. Thermogravimetry (also knows by acronym "TG" and obsolete names thermo-gravimetry thermogravimmetry is a branch of Physical chemistry, Materials research

Water vapor pressure

Main article: Vapor pressure of water

Water, like all liquids, starts to boil when its vapor pressure reaches its surrounding pressure. Vapour pressure of water can be used in many experiments particularly experiments relating to gases 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

where the temperature T_b is the boiling point temperature in degrees Celsius and the pressure p is in pascals. Water is a common Chemical substance that is essential for the survival of all known forms of Life. The Celsius Temperature scale was previously known as the centigrade scale. One gets the vapor pressure by solving this equation for p.

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

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 one (arguably the first reliable amongst many equations that have been proposed to estimate the saturation water vapor pressure at a given temperature

Vapor pressure of mixtures

Raoult's law gives an approximation to the vapor pressure of mixtures of liquids. Established by François-Marie Raoult, Raoult's law states the Vapor pressure of an Ideal solution is dependent on the vapor pressure of each 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:

where p is vapor pressure, i is a component index, and χ is a mole fraction. Fugacity is a measure of a Chemical potential in the form of 'adjusted pressure The word index is used in variety of senses in Mathematics. In perhaps the most frequent sense an index is a Superscript In Chemistry, the mole fraction of a component in a Mixture is the relative proportion of molecules belonging to the component to those in the mixture The term p_iχ_i 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). The Van der Waals equation is an Equation of state that can be derived from a special form of the potential between a pair of molecules (hard-sphere repulsion

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. An azeotrope (pronounced 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.

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

Usage of the term vapor pressure in meteorology

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,^[3] and the equilibrium vapor pressure is specified as such. Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary 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 General properties of water vapor Evaporation/sublimation Whenever a water molecule leaves a surface it is said to have evaporated 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. Brine (lat saltus) is Water saturated or nearly saturated with Salt (NaCl ^[4]

See also

• Absolute humidity
• Clausius-Clapeyron Equation
• Partial pressure
• Relative humidity
• Relative volatility
• Triple point
• Vapor-liquid equilibrium
• Vapor Pressure of Water at Various Temperatures
• Volatility

References

External links

• MSDS Vapor Pressure
• Hyperphysics
• Online vapor pressure calculation tool (Requires Registration)