At the 19th century, John Dalton studied the volume of steam which was necessary to saturate the air. He noticed that this volume depended much on the temperature.

Cases of pure substances liquid

When with balance one has a pure substance in liquid form with gas a “sky” (atmosphere above the liquid) made same pure substance, part of the atmosphere is made up of the pure substance in gas form; the contribution to the Pression of this fraction (or pressure partial of the gas pure substance) is the saturating steam pressure. So then the pressure passes in lower part of the saturating steam pressure (by a relaxation, or then by increasing the saturating steam pressure, i.e. while heating), the liquid passes violently in gas form: it is the Vaporisation, or boiling.

If this vapor is mixed with other gases, it is considered that the saturating pressure partial of vapor is the same one as the pressure of balance of the vapor alone. One can have the case where the saturating steam pressure is higher than the partial Pression gas, but lower than the total pressure: the liquid passes then slowly in gas form, it is the evaporation. There is boiling (Vaporisation) only if the saturating steam pressure is higher than the total pressure.

Thus, for a given pressure, the Point of boiling of a substance is the temperature to which the steam pressure saturating with this substance is equal to the total environmental pressure.

Starting from a situation of balance p_sat ( T ) = p_vap :

• modification of the room temperature:
• in lower part of this temperature, the vapor liquefies;
• above this temperature, the liquid evaporates, and with the top of the melting point, it vaporizes (it end).
• modification of the environmental pressure:
• above this pressure, the vapor liquefies in liquid;
• in lower part of this pressure, the liquid evaporates, and for the very low pressures, it vaporizes (it end); the limiting pressure of boiling is that for which the point of boiling is the current temperature.

In short:

• p_sat ( T ) > p_amb : boiling, Vaporization
• p_amb > p_sat ( T ) > p_vap : evaporation
• p_sat ( T ) = p_vap : balance
• p_vap > p_sat ( T ): Liquefaction

For example, a water puddle pool evaporates with the Sun (the steam pressure saturating with water is higher than the pressure partial of the vapor but lower than the atmospheric pressure). When one carries a water pan to 100  °C under an atmosphere, water boils (the saturating steam pressure exceeds the atmospheric pressure). In a bottle of gas liquefied (standard Propane or Butane), the gas boils cold in the bottle with the fur-and-with-measurement which one taps of gas (during racking, the total pressure becomes lower than the saturating steam pressure).

Mix pure substances

The Loi of Raoult governs roughly the steam pressure saturating with mixtures of liquids.

In a reasonable enclosure of size of bottle type or gas tank, a mixture of gas can be regarded as homogeneous: the thermal energy of agitation is higher than the difference in potential energy of gravity, the stratification is negligible. Thus, the mix design is almost constant beginning at the end of pulling. It can on the other hand occur a selective condensation which varies the mix design (one of gases condenses whereas the other remains in gas form, or when the two gases are liquefied the composition of the liquid differs from that of gas). Thus, the bottles of mixture have an instruction of temperature of storage to ensure that the composition of gas is well the nominal composition.

Explosive decompression

In the case of a body contained in a tight enclosure, it can occur a boiling-explosion (or Bleve ) by rupture of the enclosure when:

• under the conditions of temperature and external pressure, the body is in gas form,
• the pressure is higher in the enclosure, which maintains part of the body in liquid form.

Case of a solid pure substance

The steam pressure saturating with the majority of the solids is in general very low for “reasonable” temperatures. For example, the steam pressure saturating with the Iron to 20  °C is so weak that it is improbable to have only one Atome iron in a part although steel present is with balance with its vapor (one goes there into quantum considerations of Probabilité of presence).

In addition, the cohesion of the atoms of a solid makes that it is unthinkable to have a phenomenon of boiling. However, it can be formed Pore S in material, and inside this porosity, a solid part passes in gas form (with the same precaution as above). However, this is not related on the atmospheric pressure as for the liquids (except for extreme pressures), but rather to the properties of material, like the presence of specific defects in a Cristal (gap S) and the surface Tension.

Starting from a situation of balance p_sat ( T ) = p_vap

• modification of the room temperature:
• in lower part of this temperature, the vapor condenses in solid;
• above this temperature, the solid sublimates, i.e. it directly transforms solid into vapor without passing by a liquid phase;
• modification of the environmental pressure:
• above this pressure, the vapor condenses in solid;
• in lower part of this pressure, the solid sublimates.

Thus, for a given pressure, the Point of sublimation of a substance is the temperature to which the steam pressure of this substance is equal to the environmental pressure.

There are thus only three cases:

• p_sat ( T ) > p_vap : sublimation
• p_sat ( T ) = p_vap : balance
• p_vap > p_sat ( T ): Condensation

Saturating steam pressure and other phase shifts

The steam pressure saturating with a substance with liquid phase can be - and is generally - different from the steam pressure of this same substance in solid phase. If the temperature is such as the steam pressure of the liquid is higher than that of the solid, the liquid will vaporize but the vapor will condense in a solid, i.e. the liquid will freeze. If the temperature is such as the steam pressure of the liquid is lower than that of the solid, the solid will vaporize but the vapor will condense in a liquid, i.e. the solid will melt.

At the temperature to which two steam pressures are equal, there exists a balance between the solid and liquid phases. This temperature is mentioned like the melting point.

Calculation of the saturating steam pressure

The approximate calculation of the saturating steam pressure can be done using a formula resulting from the equation of Clapeyron, while taking as assumptions - inter alia - which the vapor behaves like a perfect gas and that the enthalpy of vaporization does not vary with the temperature in the beach considered.

$\backslash \; ln\; \backslash \; frac\; \{p\_\; \{sat\}\}\; \{p\_0\}\; =\; \backslash \; frac\; \{M.L\_v\}\; \{R\}\; (\backslash \; frac\; \{1\}\; \{T\_0\}\; -\; \backslash \; frac\; \{1\}\; \{T\})$

with:

• T₀: boiling point of the substance to a pressure p₀ given, in K
• p_sat: saturating steam pressure, in the same unit as p₀
• M: mass molar substance, in kg/mol
• L_v: latent heat of vaporization of the substance, in J/kg
• R: constant of perfect gases, equalizes to 8,31447 J/K/mol
• T: temperature of the vapor, in K

For water, for example:

• M = 0,018 kg/mol
• L_v = 2,26×10⁶ J/kg
• p₀ = 1013 mbar
• T₀ = 373 K

Steam pressure saturating with the Water in the Air

In the case of the Water in the Air, the demonstrations of this phenomenon are numerous: appearance of “vapor” (actually of the small water droplets) above a pan warm water, formation of Mist, Dew, White frost, Fog, Drizzle or Cloud S.

The steam pressure saturating with the humid air represents the maximum quantity of steam which the air can contain. It increases with the temperature. One often speaks about the Relative humidity of the air: it is about the relationship between the pressure partial of vapor and the saturating steam pressure. This rate is expressed as a percentage.

$\{\backslash \; rm\; humidit\; \backslash \; acute\; \{E\}\; \backslash \; relative\}\; =\; \backslash \; frac\; \{p\_\; \{vap\}\}\; \{p\_\; \{sat\}\; (T)\}\; \backslash times\; 100\; \backslash \%$

The formula of Rankine takes again the preceding one with coefficients slightly different (variation from 0,39 with 4,1  % on the beach of 5 with 140  °C compared to the thermodynamic tables):

$\backslash \; ln\; p\_\; \{sat\}\; =\; 13,7-\; \backslash \; frac\; \{5120\}\; \{T\}$

with:

• p_sat: steam pressure saturating with water, in atmosphere
• T: absolute temperature, in K

For more raised temperatures, one will be able to use the Formule of Duperray (variation from 0,12 to 7,7% on the beach of 90 with 300  °C):

$p\_\; \{sat\}\; =\; (\backslash \; frac\; \{T\}\; \{100\})\; ^4$

with:

• p_sat: steam pressure saturating with water, in atmosphere
• T: temperature, in °C

There exist other models like Formule of Dupre.

See too

• Pressure

• Pressure partial
• Steam pressure
• Steam pressure saturating
• evaporation
• relative Humidity
• absolute Humidity
• Psychrometry
• Hygroscopy
• Water content
• Equation with Antoine