Question

Water in an open beaker evaporates over time. As the water is evaporating, is the vapor pressure increasing, decreasing, or staying the same? Why?

Short answer

As water in an open beaker evaporates, its vapor pressure decreases. This is because water molecules escape into the air, lowering the pressure at the surface, and the temperature of the remaining liquid decreases due to evaporative cooling, which further lowers the vapor pressure.

Step-by-step solution

Define Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. This pressure is a measure of the tendency of the molecules to escape from the liquid or solid state to the gas phase at a given temperature. The stronger the intermolecular forces, the lower the vapor pressure will be.

Understand Evaporation

Evaporation is a phase transition process during which a liquid becomes a gas. In an open beaker, as evaporation occurs, some liquid molecules absorb enough energy to escape from the liquid phase to the vapor phase. This process is independent of the atmospheric pressure acting on the liquid, as the beaker is open to the air.

Effect of Evaporation on Vapor Pressure in an Open Beaker

In the case of an open beaker, the system is not closed, and therefore, vapor pressure is not maintained at an equilibrium with the condensed phase. As water molecules evaporate, they escape from the beaker into the air, and vapor pressure at the surface decreases. Furthermore, the evaporative cooling effect occurs, where the liquid molecules with higher kinetic energy (those with the most motion) are the ones that escape into the vapor phase, leaving the remaining liquid molecules with lower average kinetic energy. As a result, the liquid's temperature decreases, which in turn decreases its vapor pressure.

Conclusion

As water in an open beaker evaporates, its vapor pressure decreases. The reason for this is that water molecules escape into the air, lowering the pressure at the surface, and the temperature of the remaining liquid decreases due to evaporative cooling, further lowering the vapor pressure.