The vapour pressure of a given liquid will decrease if : (a) surface area of liquid is decreased (b) the volume of liquid in the container is decreased (c) the volume of the vapour phase is increased (d) the temperature is decreased

The concept of thermodynamic equilibrium is pivotal when discussing vapour pressure. Thermodynamic equilibrium in the context of vapour pressure refers to the state where the rate of evaporation of a liquid equals the rate of condensation from its vapor. This balance occurs at a specific temperature and pressure in a closed system. At this point, the number of molecules escaping the liquid to become vapour is the same as the number of vapour molecules returning to the liquid state.

For instance, in a closed container half-filled with water, water molecules evaporate from the surface and build up as vapor in the space above the liquid. Once the system reaches equilibrium, the air becomes saturated with water vapor, and an equal number of molecules condense back into the liquid. The pressure exerted by the saturated vapor at this moment is the vapour pressure. This pressure remains constant as long as the temperature doesn't change. If the temperature decreases as proposed in the exercise, the equilibrium shifts because fewer molecules have sufficient kinetic energy to evaporate, leading to a lower vapour pressure.

The kinetic energy of molecules is directly related to the temperature of a substance and is a crucial factor in determining vapour pressure. Essentially, when we talk about the kinetic energy of molecules in a liquid, we are referring to how fast the molecules are moving. Molecules in a liquid have various speeds, but only those with enough speed and energy can overcome the intermolecular forces holding them in the liquid and escape into the vapor phase.

At higher temperatures, the average kinetic energy of molecules increases, causing more molecules to evaporate and leading to a higher vapour pressure. Conversely, when the temperature is decreased as suggested in the exercise, the average kinetic energy is reduced. This means fewer molecules will have the energy needed to become vapor, which subsequently lowers the vapour pressure. This relationship between temperature and vapor pressure can be observed with liquids in our everyday lives, such as when a liquid evaporates more quickly on a hot day compared to a cooler one.

An intensive property is a characteristic of a substance that does not depend on the amount of the substance present. Vapour pressure is an excellent example of an intensive property. It remains constant for a given substance at a specific temperature, irrespective of the amount of liquid in the container.

As students, you may be asked why changing the volume of the liquid as in option (b) or the surface area as in option (a) does not affect the vapour pressure. The answer lies in the nature of vapour pressure as an intensive property. Whether you have a large amount of the liquid or a small one, the vapour pressure at a given temperature will be the same, provided the system is closed and has reached thermodynamic equilibrium. This demonstrates why changes in volume or surface area do not alter the vapour pressure, which can be a subtle yet fundamental concept to grasp in the study of phase equilibrium and physical chemistry.