Evaporation

Evaporation is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state.

The thermal motion of a molecule must be sufficient to overcome the surface tension of the liquid in order for it to evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore proceeds more quickly at higher temperature and in liquids with lower surface tension. Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases.

If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. When the process of escape and return reaches an equilibrium, the vapor is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur.

Gas has less order than liquid or solid matter, and thus the entropy of the system is increased, which always requires energy input. This means that the entropy change for evaporation (ΔH_evaporation) is always positive.

Forced evaporation is a process used in the separation of mixtures, in which a mixture is heated to drive off the more volatile component (e.g. water), leaving behind the dry, less volatile, component.

Factors influencing evaporation

• Concentration of the substance evaporating in the air. If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
• Concentration of other substances in the air. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
• Temperature of the substance. If the substance is hotter, then evaporation will be faster.
• Flow rate of air. This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. In addition, molecules in motion have more energy than those at rest, and so the stronger the flow of air, the greater the evaporating power of the air molecules.
• Inter-molecular forces. The stronger the forces keeping the molecules together in the liquid of solid state the more energy that must be input in order to evaporate them.

See also: Heat of vaporization, Evapotranspiration, flash evaporation, crystallization.