VAPOUR PRESSURE ANIMATION: gas molecules are shown above the surface of a liquid. They are in a closed container that looks like an aquarium. The gas molecules exchange places with the liquid molecules and the system has reached a steady state.
EXPLANATION: in a fluid (liquid or gas) the particles have different energies. It is the most energetic (fastest) molecules that manage to escape from the clutches of the liquid. In doing so, they change state and become a gas or vapour (vapor) i.e. they EVAPORATE. As they career around in the container, some of them dive back into the liquid and are captured. That is, they CONDENSE back into the liquid phase. In doing so, their energy is transferred to neighbouring molecules. In such a closed vessel, these two processes of leaving and returning eventually balance out and there is no NET flow of molecules between liquid and gas. They have reached equilibrium. Because the molecules are moving dynamically between the two phases it is called a DYNAMIC EQUILIBRIUM. The gas or vapour phase exerts a pressure on the walls of the container. This pressure is characteristic for the chemical and is called the saturated vapour pressure. Saturated just means totally full i.e. the space above the liquid is as full as it can get with evaporated molecules.