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Evaporation and Condensation
This simulation illustrates the processes of condensation (evaporation) of water vapors at boiling temperature during isothermal compression (expansion). It also demonstrates the dependence of boiling temperature on the external pressure. Gas parameters are shown numerically and graphically through the usage of (P,V) diagram.

Any substance can be transformed from one physical state to another. There are three physical states that commonly occur in nature: solid, liquid, and gaseous state. Transition between these states is referred to as a phase change. Evaporation and condensation are examples of phase changes. Any real gas can be turned into a liquid under certain conditions. This accounts for the fact that isotherms of a real gas are noticeably different from the isotherms of an ideal gas.

A liquid and its vapor in a closed vessel can coexist in dynamic equilibrium when the number of molecules leaving the liquid is the same as the number of molecules returning to the liquid from the vapor. Such a system is known as a two-phase system. The vapor that is in equilibrium with its liquid is known as saturated vapor.

The pressure of a saturated vapor is a function of temperature only, and is independent of the vapor's volume. Therefore, an isotherm of a real gas on the (P,V) diagram exhibits a horizontal section that corresponds to the two-phase state.

A liquid's boiling temperature strongly depends on the pressure. The process of water boiling in an open vessel is characterized by the quickly increasing amount of vapor bubbles that originate throughout the volume of water, and float to the surface. Boiling starts at the temperature when the saturated vapor pressure Ps(T) in the bubbles is equal to the pressure in liquid. At normal pressure P = 1.01∙105Pa = 1atm, water boiling temperature equals to 373 K (100oC).

While mountain climbing, one can notice that as the atmospheric pressure decreases with altitude, the water boiling temperature decreases as well (by approximately 1o per every 300 meters of altitude). At the elevation of 7000 m, the atmospheric pressure is about 0.4∙105 Pa, and the water boiling temperature is 70oC.

Liquids cannot boil in closed vessels, because in that case, equilibrium between the liquid and its saturated vapor occurs at every temperature. Boiling temperature at any given pressure can be found by using the equilibrium curve Ps(T).

Modify the temperature T of the process, and observe the ensuing changes on the dependency graph p(V). (Remember that the horizontal part of the graph is the two-phase region.)

The energy diagram displays the quantity of absorbed heat Q, the amount of work produced W, and the change ΔU of the internal energy in the process of isothermal expansion (compression) of water vapors. Note that in the two-phase region, the pressure of the vapor does not depend on its volume. This pressure is called the pressure of the saturated vapor, and it is represented on the horizontal region of the (P,V) diagram.

© OpenTeach Software, 2007