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Evaporation definition

What is evaporation?

This is a physical process where a substance goes from a liquid state to a gaseous state without having to reach its boiling point temperature: it occurs slowly and naturally when the molecules of the liquid that reach the surface have acquired enough energy to escape from the liquid mass where they were contained. Evaporation should not be confused with vaporization, which is the change from a liquid to a gaseous state through any means, either by evaporation or boiling.

Evaporation is one of the phases of the water cycle.

Why does evaporation occur?

A liquid’s molecules each have a certain kinetic energy that causes them to collide and gain and lose energy, which they give to their neighboring molecules. At a certain point, the most charged molecules that make it to the surface will have enough kinetic energy to overcome the force of intermolecular attraction, breaking the surface tension of the liquid and escaping into the atmosphere in the form of steam or gas. By doing so, the surface temperature is reduced.

What factors influence evaporation?

Several factors, such as temperature, atmospheric pressure, and the liquid’s exposed surface, contribute to evaporation:

  • Temperature: although it is not necessary for the average temperature of the liquid to reach its boiling point for it to begin to evaporate, it is true that there will be more molecules with enough kinetic energy to escape as vapor from the surface at a higher temperature. 
  • Atmospheric pressure: the less atmospheric pressure is exerted on the surface of the liquid, the easier it will be to release the molecules from its surface.
  • Surface: the evaporation rate is directly related to the amount of exposed surface of the liquid; this means that 300 mL of liquid in a shallow puddle evaporates much faster than 300 ml contained in a long, narrow container.

What is the difference between evaporation and boiling?

  • Evaporation is slow and occurs naturally at any temperature, while boiling occurs at a specific temperature for each liquid in a rapid heating process.
  • In evaporation, only the molecules on the surface are vaporized, while a boiling vapor is produced throughout the entire volume of the liquid.
  • With evaporation, there is a cooling process of the liquid when the most charged molecules escape in the form of gas. In boiling, a heat supply is required to maintain the boiling point of the liquid.
  • Evaporation is not readily noticeable, though in certain cases, with very large bodies of water and at certain times of heat, the vapor that rises off the surface of the liquid into the atmosphere can be seen. Boiling is a vigorous process where the bubbles that turbulently escape from the liquid are clearly evident.

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