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Concept
Vapour pressure of water
Summary
The vapour pressure of water is the pressure exerted by molecules of water vapor in gaseous form (whether pure or in a mixture with other gases such as air). The saturation vapour pressure is the pressure at which water vapour is in thermodynamic equilibrium with its condensed state. At pressures higher than vapour pressure, water would condense, whilst at lower pressures it would evaporate or sublimate. The saturation vapour pressure of water increases with increasing temperature and can be determined with the Clausius–Clapeyron relation. The boiling point of water is the temperature at which the saturated vapour pressure equals the ambient pressure.
Calculations of the (saturation) vapour pressure of water are commonly used in meteorology. The temperature-vapour pressure relation inversely describes the relation between the boiling point of water and the pressure. This is relevant to both pressure cooking and cooking at high altitude. An understanding of vapour pressure is also relevant in explaining high altitude breathing and cavitation.
There are many published approximations for calculating saturated vapour pressure over water and over ice. Some of these are (in approximate order of increasing accuracy):
Here is a comparison of the accuracies of these different explicit formulations, showing saturation vapour pressures for liquid water in kPa, calculated at six temperatures with their percentage error from the table values of Lide (2005):
{| class="wikitable"
|- align="center"
! T (°C) !! P (Lide Table) !! P (Eq 1) !! P (Antoine) !! P (Magnus) !! P (Tetens) !! P (Buck) !! P (Goff-Gratch)
|- align="center"
| 0 ||0.6113||0.6593 (+7.85%)||0.6056 (-0.93%)||0.6109 (-0.06%)||0.6108 (-0.09%)||0.6112 (-0.01%)||0.6089 (-0.40%)
|- align="center"
| 20 ||2.3388||2.3755 (+1.57%) ||2.3296 (-0.39%) ||2.3334 (-0.23%)||2.3382 (+0.05%)||2.3383 (-0.02%)||2.3355 (-0.14%)
|- align="center"
| 35 ||5.6267||5.5696 (-1.01%) ||5.6090 (-0.31%) ||5.6176 (-0.16%)||5.6225 (+0.04%)||5.6268 (+0.00%)||5.6221 (-0.08%)
|- align="center"
| 50 ||12.
Official source
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