AN607

16

Rev. 1.7

5.3. Evaporation

The average energy of molecules in a liquid is directly proportional to temperature. The vapor pressure of water will

increase as it is heated. When it reaches 100 °C at sea level, the vapor pressure will equal the atmospheric

pressure (approximately 760 Torr or 101.325 kPa) and the water will boil. The energy (vapor pressure) of water

molecules in an open partially filled container of liquid water (or ice), Figure 11, is such that some of the molecules

have enough energy to escape the attractive forces holding the water together and evaporate into the atmosphere.

Eventually all of the water will evaporate assuming the air is not saturated. If the container is sealed, Figure 12,

water molecules will evaporate into the air space above the water and some of the evaporated water molecules will

condense back into the liquid water. The rate of evaporation will exceed the rate of condensation until the air above

the water becomes saturated with water vapor. Once the air is saturated the rates of evaporation and condensation

will be equal (the vapor pressure will equal the saturation pressure inside the closed container). Evaporation takes

place at the surface of the liquid while boiling can take place throughout the volume of the liquid.

Figure 11. Water in Open Container

Evaporation > Condensation

Humidity in the open air is increasing