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The saturated vapor pressure is the basic condition for the equilibrium of closed systems

Morning dew on the leaves and fogging your car's windshield, the disappearance of gasoline from long-term storage tanks and the precipitation of long-awaited rain, all these processes combine a constant change in the substance of its aggregate state. With a more detailed study of the conditions for the transition of vapor-liquid and even vapor-solid bodies, it was observed that not only the temperature but also the pressure exerted an influence on their flow.

So in the section of thermodynamics among other physical concepts there appeared the definition of saturated vapor, and the saturated vapor pressure of each individual substance became the subject of study and accurate measurements. All descriptions and definitions of saturated vapor are reduced to the consideration of a certain closed system in which the substance is simultaneously in two aggregate states, vapor and liquid (sometimes a solid body), and the balance between them is achieved by the presence of a certain temperature and pressure. Their change leads to the boiling or condensation of liquids or to sublimation, desublimation of solids.

In other words, the concentration of the substance in the vapor state reaches the maximum possible value for the available temperature and pressure. The system occupies a position of a certain equilibrium, in which the amount of vapor and its properties get constant values.

The processes occurring inside the closed system are interesting in that the saturated vapor pressure is absolutely independent of the volume of the vessel. If you provoke an increase (decrease) in volume, for example, using a piston, in the first case this leads to an increase in the amount of steam, and in the second case, to condensation, the transition of steam into a liquid. It is possible to influence the saturated vapor pressure only by changing the temperature.

Able to describe the pressure of a saturated vapor formula, which can be found on the pages of even a school textbook. But a visual manifestation of the properties of saturated steam can be observed in the toy "drinking chicken", which rightfully won the title of pseudo-perpetual motion machine. Uncomplicated design of two welded together flasks, fixed in the middle of the axis as a child's swing, demonstrates how the periodic evaporation and conditioning of the in-air ether leads the toy into a constant movement. Saturated steam pressure receives periodic pulses to change by lowering (cooling) the toy's head in a container with water.

But the main practical application of the knowledge of the properties of saturated steam goes far beyond creating idle fun and toys. Instruments for measuring air humidity and dew point were widely used . Knowledge of the ways to regulate humidity allows you to achieve the most favorable microclimatic conditions in the premises. This not only plays an important role in creating favorable conditions for human life, but it is also necessary for the preservation of objects in warehouses, archives, libraries ...

These issues play a special role in the fuel and energy industry, where one has to deal with the storage and transportation of large volumes of flammable liquids and gases. The use of special valves that depress the saturated vapor pressure of the gasoline, although it leads to a partial loss of the valuable substance, but protects from the more unpleasant consequences arising from excessive fuel pressure in large containers for storage. The technology of liquefying gases is largely based on the laws and properties of saturated steam, its application not only facilitated the transportation of this type of fuel, but also made it available for use in the automotive industry. Cars with HBO not only save the financial assets of their owners, but also have a beneficial effect on the environment.

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