Vapour Pressure

All liquids exhibit a tendency for evaporation. Evaporation takes place at the surface of a liquid. If the kinetic energy of liquid molecules overcomes the intermolecular force of attraction in the liquid state then the molecules from the surface of liquid escape into space above the surface. The process is called ‚evaporation‘. If evaporation is carried out in a closed container system then the vapours of liquid remain in contact with the surface of a liquid. Like gas molecules vapour of molecules also execute continuous random motion. During these motions, molecules collide with each other and also with the walls of the container, losses their energy and returns back to liquid state. This process is called as ‚condensation‘.
Evaporation and condensation are continues processes. Hence, after some time an equilibrium is established, at a constant temperature between evaporation and condensation. At equilibrium number of molecules in vapour state remains constant at a constant temperature.
„The pressure exerted by vapours of liquid on the surface of liquid when equilibrium is established between liquid and it’s vapour is called VAPOUR PRESSURE of liquid.“
The vapour pressure of the liquid depends on the nature of the liquid and temperature. With increase of intermolecular force of attraction vapour pressure of liquid decrease and with rise of temperature vapour pressure of liquid increases.
In general, the higher the vapour pressure of a material at a given temperature, the lower the boiling point. In other words, compounds with high vapour pressures are volatile, forming a high concentration of vapour above the liquid; this can sometimes pose a fire hazard.

The matter of vapor pressure is important relating to the function and operation of gasoline-powered, especially carbureted, vehicles. High levels of vaporization are desirable for winter starting and operation and lower levels are desirable in avoiding vapor lock during summer heat. Fuel cannot be pumped when there is vapor in the fuel line (summer) and winter starting will be more difficult when liquid gasoline in the combustion chambers has not vaporized. Thus, oil refineries manipulate the Reid Vapor Pressure seasonally specifically to maintain gasoline engine reliability.

The Reid vapor pressure (RVP) differs slightly from the true vapor pressure (TVP) of a liquid due to small sample vaporization and the presence of water vapor and air in the confined space of the test equipment. That is, the RVP is the absolute vapor pressure and the TVP is the partial vapor pressure