Vapour Pressure — Definition
Definition
Imagine you have a beaker of water and you cover it with a lid. What happens inside? Initially, some water molecules at the surface, which have enough kinetic energy, escape into the space above the liquid as vapour – this process is called evaporation.
These vapour molecules move randomly in the enclosed space. As more and more molecules evaporate, the concentration of vapour molecules above the liquid increases. Eventually, some of these vapour molecules, moving randomly, will collide with the liquid surface and get trapped, returning to the liquid phase – this process is called condensation.
At the very beginning, evaporation is much faster than condensation. But as the number of vapour molecules increases, the rate of condensation also increases. Eventually, a point is reached where the rate at which molecules evaporate from the liquid surface becomes exactly equal to the rate at which vapour molecules condense back into the liquid.
At this point, even though individual molecules are still constantly moving between the liquid and vapour phases, the net number of molecules in the vapour phase remains constant. This state is called dynamic equilibrium.
When this dynamic equilibrium is established, the vapour molecules in the space above the liquid exert a certain pressure on the walls of the container and on the surface of the liquid. This pressure is what we define as vapour pressure.
It's important to understand that vapour pressure is a characteristic property of a liquid at a specific temperature. It does not depend on the amount of liquid present, nor on the surface area of the liquid, as long as there is enough liquid to establish equilibrium.
It primarily depends on two factors: the nature of the liquid (specifically, the strength of its intermolecular forces) and the temperature. Stronger intermolecular forces mean molecules are held more tightly, making it harder for them to escape, resulting in lower vapour pressure.
Higher temperatures provide more kinetic energy to molecules, enabling more of them to escape, thus increasing vapour pressure.