Definition Of Vapor Pressure

Definition Of Vapor Pressure

Vapor pressure exerted by saturated vapor pressure over liquid which is in equilibrium with the liquid is known as vapour pressure OR
Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system

More About Vapor Pressure

  • Vapour pressure exerted by the vapours in equilibrium with liquid is known as equilibrium vapour pressure or saturated vapour pressure
  • Temperature must be mentioned while reporting vapour pressure
  • The vapor pressure of any substance increases non-linearly with temperature
  • When a liquid is heated in an open vessel, the liquid vaporizes from the surface
  • At the temperature at which vapour pressure of the liquid becomes equal to the external pressure, vaporization can occur throughout the bulk of the liquid and vapours expand freely into the surroundings
  • The condition of free vaporization throughout the liquid is called boiling
  • The temperature at which vapour pressure of liquid is equal to the external pressure is called boiling temperature at that pressure
  • The atmospheric pressure boiling point of a liquid is the temperature at which the vapor pressure equals the ambient atmospheric pressure
  • At 1 atm pressure boiling temperature is called normal boiling point
  • If pressure is 1 bar then the boiling point is called standard boiling 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
  • Boiling does not occur when liquid is heated in a closed vessel
  • On heating continuously vapour pressure increases
  • At first a clear boundary is visible between liquid and vapour phase because liquid is more dense than vapour
  • As the temperature increases more and more molecules go to vapour phase and density of vapours rises. At the same time liquid becomes less dense. It expands because molecules move apart
  • When density of liquid and vapours becomes the same; the clear boundary between liquid and vapours disappears. This temperature is called critical temperature


  • 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