Vapour pressure of water

The vapour pressure of water is the pressure at which steam is saturated. At higher pressures water would condense. Vapour pressure is a function of temperature. In a gas mixture saturated with water vapour, the vapour pressure is equal to the partial pressure.

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

• $P = \exp(20.386-5132/T) \,$

where P is the vapour pressure (mmHg) and T is the temperature in kelvin.

• Using the Antoine equation

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

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

	A	B	C	T min. °C	T max °C
Water	8.07131	1730.63	233.426	1	100
Water	8.14019	1810.94	244.485	99	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.

Table of water vapour pressures

The following table list water vapour pressure in units of kPa and mmHg as a function of temperature in degrees Celsius.

Temperature (°C)	Vapour pressure (kPa)	Vapour pressure (mmHg)
0	0.6	4.5
3	0.8	6.0
5	0.9	6.8
8	1.1	8.3
10	1.2	9.0
12	1.4	10.5
14	1.6	12.0
16	1.8	13.5
18	2.1	15.8
19	2.2	16.5
20	2.3	17.3
21	2.5	18.8
22	2.6	19.5
23	2.8	21.0
24	3.0	22.5
25	3.2	24.0
26	3.4	25.5
27	3.6	27.0
28	3.8	28.5
29	4.0	30.0
30	4.2	31.5
32	4.8	36.0
35	5.6	42.0
40	7.4	55.5
50	12.3	92.3
60	19.9	149.3
70	31.2	234.1
80	47.3	354.9
90	70.1	525.9
100	101.3	760.0

See also

• Vapour pressure
• Gas laws
• Molar mass

Further reading

• Garnett, Pat; Anderton, John D; Garnett, Pamela J (1997). Chemistry Laboratory Manual For Senior Secondary School. Longman. ISBN 0582867649. 
• 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

External links

• Web Page listing various vapour pressure equations