When a partially filled container of liquid is sealed with a stopper, some liquid molecules at the surface evaporate into the vapor phase . However, the vapor molecules cannot escape from the container and so after a certain amount of time, the space above the liquid reaches a point where it cannot hold any more vapor molecules. Now some of the vapor molecules condense back into a liquid. The system reaches the point where the rate of evaporation is equal to the rate of condensation. This is called a dynamic equilibrium between the liquid and vapor phases.

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