how can we explain evaporation (to get 5marks)

• It is the process in which atoms or molecules in a liquid state gain sufficient energy to enter the gaseous state.
• It is the opposite process of condensation.
• The thermal motion of a molecule of liquid must be sufficient to overcome the surface tension and evaporate, that is, its kinetic energy must exceed the cohesion forcess at the surface.
• Hence, evaporation proceeds more quickly at higher temperature, at higher flow rates between the gaseous and liquid phase and in liquids with lower surface tension (i.e. higher vapor pressure).
• As 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, since the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases.
• This phenomenon is called as evaporative cooling.
• This is the reason why evaporating sweat cools the human body.

Evaporation can be shown using following diagram: