If we prepare a solution i.e., we dissolve small amount of solute in excess of solvent. We see that on the surface of the liquid of solution, number of solvent molecules became less bacause some molecules of solute are also available on the surface of the liquid. Hence vapour pressure will be decreased i.e., vapour pressure of the solvent in the solution will be less than that of the pure solvent.
Pure solvent Solution Vapour pressure= P0 Vapour pressure= PsIf P
0 be the vapour pressure of solvent in pure state and P
s be the vapour pressure of the solvent in impure state i.e., vapour pressure of the solution, then it will always exist that,
P0 > PsAt constant temperature the vapour pressure of pure liquid will always be constant. When the liquid is in equilibrium with vapours. If Kp is the equilibrium constant for this equilibrium
Kp = P0Kp only depends upon temperature.
Therefore vapour pressure of the liquid is also constant and depends only on temperature but it does not depend upon the surface area or nature of the vessel.