Vapour pressure of water

The vapor pressure of water is the pressure at which water vapor is in thermodynamic equilibrium with its condensed state. At higher pressures water would condense. The water vapor pressure is the partial pressure of water vapor in any gas mixture in equilibrium with solid or liquid water. As for other substances, water vapor pressure is a function of temperature and can be determined with Clausius–Clapeyron relation.

Vapour pressure of water (0–100 °C)^[1]

T, °C	T, °F	P, kPa	P, torr	P, atm
0	32	0.6113	4.5851	0.0060
5	41	0.8726	6.5450	0.0086
10	50	1.2281	9.2115	0.0121
15	59	1.7056	12.7931	0.0168
20	68	2.3388	17.5424	0.0231
25	77	3.1690	23.7695	0.0313
30	86	4.2455	31.8439	0.0419
35	95	5.6267	42.2037	0.0555
40	104	7.3814	55.3651	0.0728
45	113	9.5898	71.9294	0.0946
50	122	12.3440	92.5876	0.1218
55	131	15.7520	118.1497	0.1555
60	140	19.9320	149.5023	0.1967
65	149	25.0220	187.6804	0.2469
70	158	31.1760	233.8392	0.3077
75	167	38.5630	289.2463	0.3806
80	176	47.3730	355.3267	0.4675
85	185	57.8150	433.6482	0.5706
90	194	70.1170	525.9208	0.6920
95	203	84.5290	634.0196	0.8342
100	212	101.3200	759.9625	1.0000

Approximation formula

The saturated vapor pressure of water may be approximated by the following relations (in order of increasing accuracy):

• $P\text{ (mmHg)} = \exp\left(20.438 - \frac{5044}{T\text{ (K)}}\right),$

where P is the vapor pressure in mmHg and T is the temperature in kelvins.

• Using the Antoine equation

$\log_{10}P = A - \frac{B}{C + T},$

where the temperature T is in degrees Celsius and the vapor pressure P is in torr. The constants are given as

A	B	C	Tmin, °C	Tmax, °C
8.07131	1730.63	233.426	1	99
8.14019	1810.94	244.485	100	374

Graphical pressure dependency on temperature

Vapour pressure diagrams of water; data taken from Dortmund Data Bank. Graphics shows triple point, critical point and boiling point of water.

See also

• Dew point
• Gas laws
• Molar mass
• Goff-Gratch equation

References

1. David R. Lide, ed. (2005). CRC Handbook of Chemistry and Physics. Boca Raton, Florida: CRC Press.

Further reading

• Garnett, Pat; Anderton, John D; Garnett, Pamela J (1997). Chemistry Laboratory Manual For Senior Secondary School. Longman. ISBN 0-582-86764-9.
• Murphy, D. M. and Koop, T. (2005): Review of the vapour pressures of ice and supercooled water for atmospheric applications, Quarterly Journal of the Royal Meteorological Society 131(608): 1539–1565. doi:10.1256/qj.04.94
• James G. Speight : Lange's Handbook of Chemistry ISBN 0071432205

External links

• Web Page listing various vapour pressure equations