Everything about Vapor Pressure totally explained
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. 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). At any given
temperature, for a particular substance, there's a
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 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
Kelvin equation shows how equilibrium vapor pressure depends on droplet size.
An example is
water vapor when air is
saturated with water vapor. It is the vapor pressure usually found over a flat surface of liquid water,
and is a
dynamic equilibrium where the rate of
condensation of
water equals the rate of
evaporation of
water. In general, the higher the temperature, the higher the vapor pressure. When
air is at the saturation vapor pressure, it's said to be at the
dew point. 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.
The international standard for saturation vapor pressure over water is given by the
Goff-Gratch equation. Another more recent
equation for
water is the
Arden Buck Equation.
Assuming absolutely clean air, if water droplets have a high curvature, which is the case when they're smaller, they require relative humidities in excess of 100% (known as
supersaturation) to be at an equilibrium vapor pressure. As droplets approach approximately
20 micrometers, they can survive at 100% relative humidity. As the droplet grows larger by collision and
coalescence, it can survive longer because its curvature becomes smoother as the droplet grows. 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). 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.
The vapor pressure of any substance increases non-linearly with temperature according to the
Clausius-Clapeyron relation. 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. 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
The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point (for example, 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. 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. 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.
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 conversions to other pressure units are:
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'll 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.
Water vapor pressure
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
where
p is vapor pressure,
i is a component
index, and χ is a
mole fraction. The term
is the vapor pressure of component
i in the mixture. Raoult's Law is applicable only to non-electrolytes (uncharged species); it's most appropriate for non-polar molecules with only weak intermolecular attractions (such as
London forces).
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.
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 isn't equilibrium, 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.
Further Information
Get more info on 'Vapor Pressure'.
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