The system is said to be in dynamic equilibrium. At this point, we could measure something called vapor pressure. Vapor pressure is the pressure exerted by evaporating molecules or gas molecules found in dynamic equilibrium with the pure liquid molecules. Now, what will happen to our system if we add a non volatile compound to our pure substance? Remember, a non volatile compound is a compound that will not evaporate. And remember, evaporation only occurs on the surface area of the liquid.