Vapor Pressure

Vapor Pressure

If an evacuated container is partially filled with a liquid, a portion of liquid evaporates to fill the remaining volume of the container with vapor. Initially the liquid evaporates and pressure exerted by vapors on the walls of the container (Vapor Pressure) increases. After some time it becomes constant, equilibrium is established between liquid phase and vapor phase. Vapor pressure at this stage is known as equilibrium vapor pressure or saturated vapor pressure. Since process of vaporization is temperature dependent; the temperature must be mentioned while reporting the vapor pressure of a liquid.

 When a liquid is heated in an open vessel, the liquid vaporizes from the surface. At the temperatures at which vapor pressure of the liquid becomes equal to the external pressure, vaporization can occur throughout the bulk of the liquid and vapor expands freely into the surroundings. The condition of free vaporization throughout the liquid is called boiling. The temperature at which vapor pressure of liquid is equal to the external pressure is called boiling temperature at that pressure. Vapor pressure of some common liquids at various temperatures is given in below figure. At 1 atm pressure boiling temperature is called normal boiling point. If pressure is 1 bar then the boiling point is called standard oiling point of the liquid. Standard boiling point of the liquid is slightly lower than the normal boiling point because 1 bar pressure is slightly less than 1 atm pressure. The normal boiling point of water is 100 C (373 K), its standard boiling point is 99.6 C (372.6 K).

At high altitudes atmospheric pressure is low. Therefore liquids at high altitudes boil at lower temperatures in comparison to that at sea level. Since water boils at low temperature on hills, the pressure cooker is used for cooking food. In hospitals surgical instruments are sterilized in autoclaves in which boiling point of water is increased by increasing the pressure above the atmospheric pressure by using a weight covering the vent.

Boiling does not occur when liquid is heated in a closed vessel. On heating continuously vapor pressure increases. At first a clear boundary is visible between liquid and vapor phase because liquid is more dense than vapor. As the temperature increases more and more molecules go to vapor phase and density of vapors rises. At the same time liquid becomes less dense. It expands because molecules move apart. When density of liquid and vapors becomes the same; the clear boundary between liquid and vapors disappears. This temperature is called critical temperature.