Evaporation is a phase transition in which a liquid becomes a gas through the escape of particles from its surface. It can occur below the liquid’s boiling point and is distinct from boiling, which involves vapor formation within the liquid. Its reverse process is condensation. Although commonly associated with water, evaporation occurs in many liquids and is important in environmental processes, cooling, and industrial operations. (openstax.org)
Molecular mechanism
Particles in a liquid are continuously moving and exchanging energy. At any given temperature, they possess a range of kinetic energies, rather than identical energies. Some particles near the surface have sufficient energy to overcome intermolecular attractions and enter the gas phase. Increasing temperature generally increases the fraction able to escape. Evaporation therefore does not require the whole liquid to reach its boiling point. (openstax.org)
The strength of intermolecular attraction helps determine a substance’s tendency to evaporate. For example, hydrogen bonding contributes to the cohesion of liquid water. Vaporization separates molecules from one another; it does not normally split a molecule into its constituent atoms or constitute a chemical reaction. Liquids with weaker intermolecular attractions generally have higher equilibrium vapor pressures at comparable temperatures. (openstax.org)
Vapor pressure and equilibrium
In a closed container, evaporated particles accumulate above the liquid. Some return to the surface and condense. If liquid remains and temperature is held constant, the two opposing rates can become equal, establishing dynamic thermodynamic equilibrium. Molecular exchange continues, but there is no net conversion between liquid and vapor. The pressure exerted by the vapor under these conditions is called the equilibrium vapor pressure. (openstax.org)
For a pure liquid with a macroscopically flat surface, equilibrium vapor pressure depends primarily on temperature and the substance’s identity, not on the exposed surface area. Surface area can, however, affect how quickly equilibrium is reached. In an open environment, net evaporation is favored when the surrounding vapor’s partial pressure is below the equilibrium value at the liquid’s surface temperature. Removal of vapor helps maintain this imbalance. (openstax.org)
Boiling occurs when a liquid’s equilibrium vapor pressure reaches the surrounding pressure, allowing vapor bubbles to grow within the liquid. Surface evaporation can continue at much lower temperatures. Consequently, water in an open container can gradually disappear without boiling, and a sealed vessel can retain liquid while evaporation and condensation continue simultaneously. (openstax.org)
Energy requirements and cooling
Evaporation requires energy to separate particles against their mutual attractions. The latent heat of vaporization is the energy required per unit mass for conversion from liquid to gas under specified conditions. The phase-change energy requirement is commonly written
[ Q=mL_v, ]
where (Q) is the transferred heat, (m) is the mass vaporized, and (L_v) is the specific latent heat of vaporization. Its units are joules per kilogram. For water at its normal boiling point, (L_v) is approximately (2.26\times10^6) J/kg. (openstax.org)
When energy removed by evaporation is not fully replaced, the liquid and nearby surroundings cool. This is evaporative cooling, the physical basis of cooling through sweat evaporation and of many cooling towers. Continued evaporation need not produce continued temperature decline: incoming heat can compensate for the energy carried away by vapor. The temperature behavior therefore depends on the balance between energy supply and evaporative loss. (openstax.org)
Factors controlling the rate
Environmental evaporation depends on both the available energy and the transport of vapor away from the surface. Important controls include solar radiation, surface and air temperatures, humidity, and air movement. Drier air generally permits greater net water evaporation under otherwise comparable conditions, while wind carries vapor away and reduces its accumulation near the surface. These factors interact, so temperature alone does not determine the rate. (fao.org)
Water availability is another essential constraint. A wet soil surface may initially evaporate at a rate largely controlled by weather. As the surface dries, the movement of moisture from deeper soil can become insufficient to maintain that rate. Evaporation then becomes limited by water supply, even when the atmosphere remains dry and energy is available. Shading by vegetation also reduces the radiation reaching the soil. (fao.org)
Environmental significance
Evaporation transfers water from oceans, lakes, rivers, wet soils, and other surfaces into the atmosphere, making it a major component of the water cycle. Atmospheric transport redistributes this vapor, and condensation and precipitation return water to the surface. Solar energy supplies much of the energy driving this circulation. (usgs.gov)
Water loss through plants is called transpiration. It includes vaporization within plant tissues and release of vapor, predominantly through leaf openings. The combined transfer through evaporation and transpiration is evapotranspiration, an important quantity in agricultural water accounting. The relative contributions of soil evaporation and plant transpiration change as vegetation develops and covers the ground. (usgs.gov)
Measurement and applications
An evaporation pan measures water loss from an exposed water surface, usually expressed as an equivalent depth per day. Measurements account for rainfall and water added or removed. Pan results integrate weather effects, but heat storage, construction, and local surroundings make them different from evaporation over larger water bodies or crops; empirical coefficients are used when estimating reference crop evapotranspiration. (fao.org)
Industrial applications exploit either evaporative cooling or the removal of water. Cooling towers dissipate waste heat by evaporating part of a circulating water supply. Evaporation ponds concentrate saline water and permit the recovery of salt and other dissolved minerals, which remain as water is lost to the atmosphere. (usgs.gov)
References
- 6 Humidity, Evaporation, and Boiling - College Physics 2eopenstax.org
- 3 Phase Transitions - Chemistryopenstax.org
- 3 Phase Change and Latent Heat - Physicsopenstax.org
- 5 Phase Changes - College Physics 2eopenstax.org
- Chapter 1 - Introduction to evapotranspirationfao.org
- Evapotranspiration and the Water Cycleusgs.gov
- CHAPTER 3: CROP WATER NEEDSfao.org
- Chapter 4 - Determination of ETofao.org