Vapor pressure

The vapor pressure is the pressure that comes when a vapor is in a closed system with the associated liquid phase in thermodynamic equilibrium. The vapor pressure increases with increasing temperature and depends on the present substance or mixture. Is in an open system, the vapor pressure of a liquid is equal to the ambient pressure, the liquid begins to boil.

  • 3.1 Practical significance


In a liquid, the particles are greater than 0 K to leave the quest, the liquid bandage at a temperature. Counteract the surface tension and atmospheric pressure. The surface tension decreases with increasing temperature and the vapor pressure rise. At the boiling point, the surface tension of zero, and the vapor pressure equal to atmospheric pressure.


Substances can occur in three states: solid, liquid and gaseous. If there is next to the gas nor the associated liquid phase, so is referred to as the gas vapor. The vapor pressure is thus the pressure in a multi-phase system.

Keeps the temperature of a closed system constant, then an equilibrium between the liquid and the gaseous phase. The gas phase has a vapor pressure. The equilibrium Provides a complete, one also speaks of saturation vapor pressure.

No liquid phase is present and only a gaseous phase, one measures no vapor pressure, but the gas pressure.

There are various substances in the considered system, the measured pressure of the gas phase of the partial pressures of the various substances is composed. Condition is that these gases behave like ideal ( Dalton 's law).

Definition in terms of the chemical

In chemistry, the vapor pressure means the partial pressure of a gas ( multicomponent system ), which is in thermodynamic equilibrium with its liquid or solid phase, it can be calculated by Raoult's law. In the one-component system, this pressure is referred to as the equilibrium pressure.

Definition in the sense of Meteorology

In meteorology refers to as vapor pressure the partial pressure of the gas (in this case usually the water vapor pressure ). The maximum vapor pressure prevailing at saturation is identical to the definition of the vapor pressure the chemical referred to as the saturation vapor pressure.

Water vapor pressure

Are there any water and water vapor in thermodynamic equilibrium side by side so the pressure is purely a function of temperature:

This temperature-dependent and material-specific pressure is known as the steam pressure, and the graph of this function as a steam pressure curve. The vapor pressure curve ends at the critical point.

On increasing the temperature steam pressure and density are rising substantially, while the density of the liquid decreases. The properties of water and steam are more similar with increasing temperature until at the critical point at T = Tc = 374.12 ° C and p = pc = 22.12 MPa, the difference is quite gone, and now exists only a single phase. When approaching the critical point the heat of vaporization disappears, and there are strong density fluctuations, identified as critical opalescence.

Practical significance

In an open pot heated water boils when its vapor pressure exceeds the ambient air pressure. The boiling point of water is so dependent, and the air pressure decreases with increasing altitude, as the air pressure of the natural ground is smaller with increasing distance from the sea level. In 2000 m altitude water boils at 93 ° C, at 8000 m already at 74 ° C.

The physical laws to steam pressure and evaporation (steam pressure curve, the Clausius- Clapeyron equation, etc.) were first examined and expressed in the context of the steam engine. Again, there is a coexistence of liquid and gas. The fact you get in the steam engine exploits the fact that the vapor pressure is independent of the volume, as long as you " liquid - gas " moves in the two-phase system. The only thing that changes at constant temperature, the ratio " liquid - gas ". The pressure in the boiler, which moves the piston, not ( change in volume of the piston movement in the cylinder ) is changed by the piston movement. The piston position must not be changed so far, however, that one of the phases would disappear.


The calculation of the saturation vapor pressure can occur, for example methods according to Lee - Kesler and Ambrose -Walton. Both methods are based on the correspondence principle, in which critical data and acentric factor are used.

Higher accuracy requirements satisfy the vapor pressure equations, use the substance-specific, adapted to experimental data parameters, such as the Wagner and the Antoine equation.

Water vapor pressure in meteorology

In meteorology, under the designation vapor pressure usually the vapor pressure of water vapor ( water vapor pressure ), and thus its partial pressure understood. The vapor density is the absolute humidity.

The vapor pressure according to the definition of meteorology, so the partial pressure of a gas within a gas mixture can be calculated by rearranging the ideal gas equation with the following formula approximation:

The individual symbols stand for the following sizes:

  • E - vapor pressure
  • ρi - Vapour density
  • Ri - individual gas constant
  • T - temperature

The individual gas constant for water vapor is 461.5 J / ( kgK ).

Since the water vapor makes up only a small part of the air pressure, arises only after a fairly long period of thermodynamic equilibrium, will result in significant sub -saturation in the atmosphere are possible without the existing liquid water boils immediately. For this reason and the high dynamics in the atmosphere thermodynamically stable states are usually rarely or only briefly encountered, especially in very active weather zones of the earth's atmosphere. Due to the locally relatively higher vapor pressure above the liquid phase (see above) grow in the simultaneous presence of water and ice in a cloud the ice particles at the expense of the water droplets.

To a solid phase nichtüberhitzten the vapor pressure is lower than a liquid phase over the same temperature. Are the two phases of the surrounding gases into contact, so thus increasing the proportion of the solid at the expense of liquid. This is because that the stronger binding of the particles in the solid and the resulting heat of fusion in the case of sublimation, ie the phase transition solid-gaseous, must also be overcome or applied. This has the consequence that it is easier and thus more likely to evaporate or evaporation of the particles of the liquid phase as the sublimation of particles on the solid phase. It is thus on the liquid to make more of the particles in the gaseous state as to the solid, which results in increasing the vapor pressure and to locally increase the solid phase.