Evaporation

Evaporation is a phase change of a liquid to a gaseous state. It occurs when molecules in the liquid have enough kinetic energy to escape the surface of the liquid - it is therefore a surface phenomena, and is not to be confused with the volume phenomena of boiling.

Evaporation is a cooling mechanism as the escaping molecules require heat from the rest of the liquid in order to change phase, thereby leaving the remaining liquid cooler. This mechanism is used by the human body in the process of perspiration (sweating) in order to cool off and can be read about on Hyperphysics.

Liquids with strong intermolecular attractive forces require more energy for evaporation to occur. Since water molecules have an asymmetric electric charge, they naturally form weak electrostatic bonds with one another known as Van der Waals bonds. These bonds mean that the amount of heat needed to evaporate water is quite significant when compared to other liquids that lack these bonds, with one gram of water requiring almost 2300 joules of energy to evaporate. This energy needed to evaporate a liquid is known as the latent heat of evaporation.

Evaporative fog above a lake in winter. This fog is the result of evaporation of water from the lake's surface, which then condenses back into liquid water when it meets the cold air.

In water sciences, evaporation is commonly lumped-in with the process of transpiration, the act of plants exchanging water vapour with the atmosphere through their stomata, and the two processes combined are known as evapotranspiration. While it is not possible in all situations to separate out the evaporative and the transpirative signal in real-world measurements, it is often more useful to deal with the combined evapotranspirative signal when trying to understand parts of the hydrologic cycle for water-policy decision making. This is expected to be of particular importance in the face of climate change, as hotter temperatures will result in a greater amount of evaporation worldwide.

Click here to learn more about how evaporation works!

For Further Reading

• Phase change
• Boiling point
• Transpiration
• Hydrologic cycle
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