Suppose we have a vessel which is closed inside. We have water now Here I am considering water is at atmospheric pressure, then we are heating this water inside a closed container by supplying heat externally. When we are adding heat to this water it will evaporate and since it is at atmospheric pressure so the temperature required for evaporation would be a hundred degrees Celsius. Now when the temperature reaches 1000C at that time water molecules will be converted into vapor and this vapor will exert a pressure on this liquid water and hence this pressure would be called as “vapor pressure”. So here we have defined vapor pressure is the pressure exerted by the vapor on a liquid.