Question

Is there vapor pressure in an open container?

Answer 1

Hint : Vapour pressure is the pressure exerted by a vapour when it is in equilibrium with the liquid or solid form of the same substance i.e., pressure at a condition when the substance can exist in both or in all three phases.

Complete Step By Step Answer:
We know that the vapour pressure is a measure of the tendency of a substance to change its phase into gaseous or vapour state. The vapour pressure of a substance increases with increase in temperature and the temperature at which the vapour pressure at the surface of a liquid becomes exactly equal to the pressure exerted by surroundings is known as the boiling point of the liquid.
Now, in the case of an open container, the air above the surface of liquid is not at thermodynamic equilibrium with the mixture of water and vapor bubbles formed while heating the liquid. The only thing that the air does is to control the overall pressure acting on the mixture which is basically 1 atm. In order for water to boil, which means the forming bubbles below the surface, the temperature has to be high enough for vapor pressure inside the bubbles to push back the atmospheric pressure so that the bubbles can grow. This happens only when the equilibrium vapor pressure matches with the atmospheric pressure that is present above the mixture.
Thus, we can conclude that there exist the vapor pressure in the open container when the water vapour in the rising bubbles is virtually at equilibrium with the liquid that surrounds it and if the liquid is taken in an open container and exposed to normal atmospheric pressure, then the liquid boils when its saturated vapor pressure becomes equal to 1 atm.

Note :
Remember that in a closed container, there will be a fixed number of the gaseous particles present in the space above the liquid. When these particles hit the walls of the container, they exert a pressure which is known as the saturated vapor pressure of the liquid.