evaporation

Evaporation is the process by which molecules at the surface of a liquid gain sufficient energy to overcome intermolecular forces and escape into the surrounding gas phase. Unlike boiling, evaporation occurs at temperatures below the liquid’s boiling point and typically happens only at the liquid’s surface.

It is an endothermic process, requiring latent heat of vaporization. The rate of evaporation depends on the temperature, the surface area of the liquid, and the difference between the liquid’s vapor pressure at the surface and the partial pressure of vapor in the air. Only surface molecules with enough energy escape; the remaining liquid cools as energy is taken away.

Ambient conditions strongly influence evaporation. Higher temperatures increase molecular energy and the vapor pressure of the liquid. A larger surface area provides more molecules at the interface. Drier air and greater air movement remove evaporated vapor more quickly, raising the rate. Humidity and air pressure also modulate the process.

Natural and technological relevance is extensive. Evaporation drives the hydrological cycle and is fundamental to drying and cooling processes. It is exploited in distillation and desalination, and in practical activities such as drying clothes or drying surfaces. Evaporation can also be observed in evaporation ponds and solar stills, which concentrate vapor or recover freshwater from saline sources. Dew and fog involve condensation, the opposite process, as air cools and moisture released from the surface condenses.

Boiling, by contrast, is rapid vaporization that occurs throughout the liquid at its boiling point when vapor pressure equals atmospheric pressure; evaporation is gradual, occurs at the surface, and can proceed below the boiling point.