Thermal Physics

Evaporation (2.1.c)

1. Define evaporation?
   Evaporation is the escape of molecules of liquids from its surface.
   

2. At what temperature evaporation occur?
   At any temperature
   

3. What effect evaporation may leave on its liquid?
   It will cool the liquid
   

4. List examples that show the cooling effect of evaporation?

   1. When the human body becomes hot, sweating occurs to cool the body by evaporation

   2. Liquids are kept in pots of porous materials to cool them by evaporation

   3. In refrigerators, the evaporation of the volatile liquid (Freon) in their tubes causes the continuous cooling.

   4. When a volatile liquid is placed on the hand, its evaporation causes the hand feel cold (local anthesia)
      

5. Explain how evaporation cools liquids?
   The molecules of a liquid are in continuous random motion having an average kinetic energy proportional to its temperature.  The most energetic molecules near the surface of the liquid can over come the attractive forces of other molecules and can escape from the surface into the air above it, thus the average kinetic energy decreases and the temperature of the liquid drops.
   

6. List the factors which increase evaporation?

   1. Increase in the temperature of the liquid

   2. Increase in the surface area of the liquid

   3. Air passing across the liquid surface
      

7. Why the increase in temperature increase evaporation?
   The increase in temperature makes the molecules move faster, so that more of them have enough energy to escape from the surface.
   

8. Why the increase in liquid surface area increase evaporation?
   Large surfaces have more fast molecules than small surfaces.  Thus, more molecules have the chance to escape from the liquid surface.
   

9. Why winds and drought  increase the rate of evaporation?
   Winds carry away the evaporated molecules from the surface and allow more molecules to evaporate.