4.1: Saturation

Evaporation and Condensation

Imagine a beaker filled halfway with water as shown in Figure \(\PageIndex{1}\). Let's put a lid on it to constrain water molecule movement and eliminate the wind's influence on evaporation. At the water surface, some molecules absorb thermal energy from the environment, undergo phase change, and enter the air as water vapor (evaporation). These molecules move about suspended in the air, but they have nowhere to go because the top is closed. Thus, a fraction of the molecules near the surface absorb onto the surface and recombine to form liquid water (condensation). As evaporation increases, the water level in the beaker decreases, as more water molecules enter the closed air above the liquid. This occurs because, at typical room temperature conditions, the number of molecules evaporating exceeds the number of water molecules condensing back onto the surface. This is called net evaporation. As the number of water vapor molecules in the air increases, so does the number of vapor molecules condensing back to the liquid phase. After some time, the amount of water entering the air from evaporation equals that condensing. When this occurs, the air is said to be Saturated.

Figure \(\PageIndex{1}\): Evaporation and condensation in an enclosed beaker of water. (CC BY 4.0; Neel Desai)

The air's saturation level is directly related to its temperature. As air temperature increases, air becomes thinner, and more water vapor molecules have more energy to remain in the gas phase. As the temperature decreases, air becomes denser, and air and water molecules slow down, increasing the likelihood that water vapor will condense onto surfaces. Thus, at higher temperatures, there is more water vapor in the air than at lower temperatures. Saturation occurs when air contains the maximum amount of water vapor possible for its given temperature. When that happens, the rate of condensation equals the rate of evaporation.

Vapor pressure at saturation

Every gas in the atmosphere exerts pressure; for example, vapor pressure contributes to the total atmospheric pressure. Since the total air pressure near sea level is nearly 1000 hPa, nitrogen (78%) exerts nearly 780 hPa, and oxygen (21%) exerts nearly 210 hPa. Specifically, for water vapor, the more it is added to the atmosphere, the higher the vapor pressure. The units for vapor pressure are the same as those for pressure and can be expressed in pascals (Pa), hectopascals (hPa), or kilopascals (kPa).

The amount of water vapor that the atmosphere can contain depends on temperature. As noted earlier, lower-temperature air contains less water vapor than higher-temperature air. If we consider this quantitatively in terms of pressure, the saturation vapor pressure is the pressure exerted by water vapor molecules moving over a liquid water surface. When the partial pressure of water vapor equals the saturation vapor pressure, the air is saturated with water vapor. Figure \(\PageIndex{2}\) shows the relationship between the saturation vapor pressure and temperature in the atmosphere. Lower temperatures are saturated with respect to water vapor at lower vapor pressures, while higher temperatures need higher vapor pressures to be saturated. Temperature is the primary factor determining the saturation of water vapor.

Figure \(\PageIndex{2}\): A graph of the saturation vapor pressure as a function of temperature showing the exponential relationship between the two from the Clausius-Clapeyron equation. (CC BY-SA 4.0; Modified from Firepulaski via Wikimedia Commons).

In Figure \(\PageIndex{2}\), notice the saturation vapor pressure value at the boiling temperature, 100 °C. The saturation vapor pressure is 101.325 kPa (1013.25 hPa), which is approximately equal to atmospheric surface pressure. Water boils at the Earth’s surface when the saturation vapor pressure is equal to the atmospheric pressure, which is why water boils at 100 °C. Will water boil at the same temperature at the top of Mount Everest?