Evaporation

Evaporation is also a process where a substance undergoes a change in state. However, it occurs in a slightly randomised manner. Therefore, we seldom use latent heat to describe this process.

Although Boiling & Evaporation both involve a change of state from liquid to gas, there are differences

• Boiling:
• • Occurs at boiling point (constant temperature);
  • Occurs throughout the liquid;
  • Bubbles can be seen

• Evaporation:
• • Occurs at any temperature;
  • Occurs at the surface of the liquid;
  • No bubbles can be seen.

• A substance with a high boiling point will have a lower rate of evaporation.
  • Liquids with a higher boiling point have stronger intermolecular bonds.

 

• Factors affecting rate of evaporation – rate of evaporation increases when
• • Temperature increases
  • Surface area increases
  • Humidity decreases
  • Wind speed increases
  • Atmospheric pressure decreases

Explanations using Kinetic Theory Why Evaporation Occurs: The liquid molecules are in constant random motion and collide with each other continuously. When the molecules with higher kinetic energy collide with other molecules when it is near the surface of the liquid, it may gain enough energy to overcome the intermolecular forces of the liquid and leave the liquid body. Why Evaporation Causes Cooling: During evaporation, the fastest-moving molecules leave the liquid. The average kinetic energy of the molecules remaining in the liquid is decreased. Since temperature is directly proportional to the average kinetic energy of the molecules in the liquid, temperature falls, and hence cooling occurs.

 

<< Back

Thermal Physics

Next >>