The vaporization and condensation processes will be introduced in this section. The concept of vapor pressure is then presented. The conditions when boiling and cavitation occur will be discussed as well.

Vaporization (Evaporation)

Condensation: Contrails

When the molecules at the surface of a liquid or solid gain enough energy to overcome the cohesive force, the molecules will escape into the air. The substance undergoes a phase change and turns into vapor. This process is referred to as vaporization (e.g., evaporation and sublimation, see below). In general, the rate of vaporization increases with the temperature.

The process of phase change from liquid to vapor is called evaporation. For example, water will evaporate into vapor when the temperature reaches 100^oC (for atmospheric pressure at sea level). A phase change directly from solid to vapor is called sublimation. An example of a sublimation process is dry ice at room temperature. The dry ice will become vapor. Also, ever wonder why moth balls will "disappear" over a period of time, but one can still smell the odor? This is another example of sublimation.

The process of phase change from a vapor to liquid is called condensation. The phenomenon of condensation can be observed in our daily lives. The contrail (condensation trail) left by an airplane is an example of the condensation process. As the hot humid exhaust air from the nozzle mixes with the surrounding air, a visible trail is formed due to condensation.

Vapor Pressure

Now consider a closed container partially filled with a liquid, as shown in the figure. As the liquid molecules at the surface gain sufficient energy to escape into the air (evaporation), some of the liquid molecules will collide with the wall or air molecules, bounce back and re-enter the liquid (condensation). Over a period of time, the system will reach a steady-state where the rate of evaporation is the same as the rate of condensation. At this instance, the pressure exerted on the liquid surface by the liquid vapor is called vapor pressure. Vapor pressure is a fluid property, and it is a function of the temperature. Generally, the vapor pressure increases with temperature. The value of vapor pressure for water and is summarized in the table below as a function of temperature.