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Steam, Gaseous Water, and Water Vapor

Yellowstone fumerole

Yellowstone fumerole

At sufficiently high-temperatures/low-pressures all particles in a gas phase obey the Ideal Gas Law.

PV = nRT


where P, V, T and n are respectively the pressure (Pa), volume (m3), temperature (K), and amount of the gas (mol) and R is the ideal gas constant (J ˣ mol-1 ˣ K-1). The Ideal Gas Law does not hold exactly at higher-pressures/lower-temperatures when the finite size of the particles and the interactions between particles have significant effects.


Gaseous water is water vapor and one of the lightest of gasses. In science and engineering, the word 'steam' is also used for water vapor, but usually when above the boiling point of water. As commonly used in the English language, 'steam' also may mean the white cloud of fine liquid water droplets of condensed water vapor that is produced by a boiling kettle or at fumeroles (see right), for example.


Water vapor is transparent to 'visible' light but absorbs strongly in the infrared. It is the main absorber of the sunlight in the atmosphere; without it, the Earth would be in a permanent ice-age. The mechanisms through which water vapor in the lower troposphere (at temperatures up to around the triple point of water; ~ 0.01 °C, ~5 km) influences convection, circulation and the formation of clouds, have been reviewed [3139].


The 13 trillion tons of water in the atmosphere (~0.33% by weight) is responsible for about 70% of all atmospheric absorption of radiation, mainly in the infrared region where water shows strong absorption. Atmospheric water contributes substantially to the greenhouse effect, more than twice that due to carbon dioxide, ensuring a warm habitable planet, but operates a negative feedback effect, due to cloud formation reflecting the sunlight away. Molecular rotations and vibrations are obtained using the spectroscopic data and summarized for all of water's isotopologues by IUPAC [3035].


As with other gasses, gaseous water is not an ideal gas, although it behaves nearly as an ideal gas at low densities, approximately obeying the relationship,

P = r ˣ R ˣ T

where P is the pressure ( Pa), r is the molar density (mol ˣ m-3 ), R is the gas constant ( J ˣ mol-1 ˣ K-1), and T is the temperature (K).


At higher densities, corrections can be applied

P/rRT= 1 + Br + Cr2 + Dr3 +...


Variation of the second virial coefficient, from [3034]

Variation of the second virial coefficient with temperature, from [3034]
The second virial coefficient (B) depends on temperature. Except at high densities, the higher virial coefficients (C, D, etc.) are usually ignored (i.e. they are put equal to zero). It can be seen right that the second virial coefficient increases considerably at lower temperatures [3034]. This indicates greater deviations from ideal behavior at lower temperatures which is partially due to dimer formation (shown blue right [3037]). The data for D20 is also shown as dotted lines with only limited data available for D20 second virial coefficient [3038].


Hydrogen bonding between water molecules occurs in the gas phase with, typically within the ambient atmosphere, over one water dimer forming for every thousand free water molecules rising to about one in twenty in steam. The dimer formation causes deviations from ideal gas behavior in gaseous water.


The thermodynamic properties of gaseous water above 100 °C, 101,325 Pa are described by the The International Association for the Properties of Water and Steam (IAPWS). Experimental data for dilute gas phase heavy water (D2O) are available for viscosity and thermal conductivity and has been calculated for molecular diffusion [3036].


The maximum water in the air, at 1 atm

The maximum water in the air varies with temperature, at 1 atm

Water is present in the atmosphere in both liquid and gaseous forms. The maximum gaseous composition depends on the atmospheric temperature (see right). The average relative humidity of the atmosphere is about 75% at ground level reducing to about at 45% at 5000 m.


When we breathe out we expire an aerosol of fine (nm - µm+ radius) water droplets plus water vapor. This aerosol has been detected by a water-cluster-detecting breath sensor developed for detecting drunk or drowsy drivers [1801].


The fugacity of water is described elsewhere.

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