Explanations:
Vapour pressure is related to the evaporation and condensation of a liquid. Before we can explain what vapor pressure is, we need to first understand what evaporation and condensation mean.
All liquids have the tendency to evaporate and this evaporation occurs at the surface of a liquid. The liquid has an intermolecular force of attraction and when the kinetic energy of the liquid molecules overcomes this force of attraction, the molecules of the liquid began to escape to the surface. This is called evaporation.
Also in a closed container, the gas molecules possess random motion. During this motion, the molecules begin to collide with the wall of the container, lose energy, and then return back to the liquid. This is known as condensation.
This evaporation and condensation is a continuous process and equilibrium is reached after some time i.e the number of molecules leaving the liquid will be equal to the molecules leaving.
When equilibrium is reached between the liquid and the vapor, the pressure exerted on the surface of the liquid at this point is known as the VAPOR PRESSURE
For instance, ice changes state to form a liquid (water), and this water evaporates at 100 degrees Celcius to form a vapor. The pressure exerted by this water vapor on the surface of the water is the vapor pressure.