All liquids, at any temperature, exert a certain vapor pressure. The vapor pressure can be thought of as the degree to which the liquid molecules are escaping into the vapor phase. The vapor pressure increases with temperature, because at higher temperature the molecules are moving faster and more able to overcome the attractive intermolecular forces that tend to bind them together. Boiling occurs when the vapor pressure reaches or exceeds the surrounding pressure from the atmosphere or whatever else is in contact with the liquid.
At standard atmospheric pressure (1 atmosphere = 0.101325 MPa), water boils at approximately 100 degrees Celsius. That is simply another way of saying that the vapor pressure of water at that temperature is 1 atmosphere. At higher pressures (such as the pressure generated in a pressure cooker), the temperature must be higher before the vapor pressure reaches the surrounding pressure, so water under pressure boils at a higher temperature. Similarly, when the surrounding pressure is lower (such as at high altitudes), the vapor pressure reaches that pressure at a lower temperature. For example, in the Denver, Colorado area of the U.S. where the elevation above sea level is approximately one mile (1600 meters), the atmospheric pressure is about 83% of a standard atmosphere, and water boils at approximately 95 degrees Celsius.
The relationship between vapor pressure and temperature (or, looked at in the reverse direction, between boiling temperature and pressure) is called the vapor pressure curve. Water's vapor pressure curve is of great importance, since it determines the relationship between temperature and pressure in any process where water is going from a liquid to a vapor state or vice-versa. Such processes are important both in industry (for example in steam power generation where water is made into steam in boilers and eventually condensed after running through turbines) and in nature (for example, in rainfall and evaporation from bodies of water).
In the following table, we list the vapor pressure for water
as a function of temperature as taken from the latest
IAPWS formulation for general and scientific use.
The first value in the table is for water's triple point, which is the thermodynamic state where vapor, liquid, and solid coexist. The last value is for water's critical point. The critical point is the end of the vapor pressure curve; there the vapor and liquid phases become identical and at higher temperatures there is only a single fluid phase.
Updated April 26, 2000