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How does a liquid exchange particles with the gas above its surface?
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liquid
Figure 1: Phase diagram of argon.
At the interface between a liquid and the gas above it, a continuous, two-way exchange of particles occurs. This process is driven by the random motion and varying kinetic energies of individual particles.1
- Evaporation: Molecules within the liquid possess a range of kinetic energies. When those near the surface attain sufficient energy to overcome the attractive forces of the surrounding liquid, they escape into the gaseous state.12
- Condensation: Simultaneously, gas molecules in the vicinity of the liquid surface may lose enough kinetic energy through collisions or cooling to be captured by the liquid, effectively transitioning from the gas phase back into the liquid state.1
In a closed system, this exchange eventually reaches an equilibrium state where the number of particles escaping the liquid equals the number of particles condensing from the gas. The pressure exerted by the vapor when it is in this state of equilibrium with the liquid is known as vapour pressure.13