Evaporation

Lore & Background

Evaporation occurs when molecules near a liquid's surface absorb enough energy to overcome vapor pressure and escape as gas. The rate is influenced by temperature, concentration of the evaporating substance in the surrounding gas, flow rate of air, surface area, pressure, and intermolecular forces. Higher temperatures and greater air movement accelerate evaporation, while high humidity slows it. On a molecular level, there is a Knudsen layer, only a few molecules thick, where the phase is undetermined, and no strict boundary between liquid and vapor is visible macroscopically.

Reader's Guide

Evaporation is fundamental to the water cycle, with solar energy driving evaporation from oceans, lakes, and soil. It is also critical in climate modeling, as the rate of evaporation of liquid water remains one of the principal uncertainties in modern climate science. The process is endothermic, absorbing heat and causing cooling, which is why evaporating sweat cools the human body. Industrial applications include many printing and coating processes. The Clausius–Clapeyron relation describes the equilibrium between vapor and liquid, linking vapor pressure to temperature and enthalpy of vaporization. Despite its importance, the mechanism for water evaporation is not completely understood, as theoretical calculations require prohibitively long computer simulations.

Did You Know?

• Evaporation occurs only when molecules are near the surface, moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces.
• The rate of evaporation in an open system is related to the vapor pressure found in a closed system.
• The photomolecular effect means that photons hitting the liquid surface can cause molecules to break free without additional heat.
• Liquids that do not evaporate visibly at a given temperature, such as cooking oil at room temperature, are still evaporating, but much more slowly.