Evaporation | Physics for GCSE/IGCSE - Year 11 PDF Download

Cooling by Evaporation

• Evaporation denotes the transition of a liquid into a gas state.
• It takes place:
  • Regardless of temperature.
  • Solely from the surface of the liquid.
• Molecules within a liquid possess varying energy levels, ranging from high to low.
  • The average energy level correlates with the liquid's temperature.
• Evaporation transpires when highly energetic molecules near the liquid's surface possess adequate energy to break free.
  • Consequently, the liquid's average energy diminishes.
  • Thus, evaporation leads to the cooling of liquids.

Temperature, Surface Area & Air Movements

These factors all affect the rate of evaporation:   

• Heightened temperature boosts the kinetic energy of liquid molecules.
  • Molecules possessing greater energy are likelier to surpass the intermolecular forces that keep them in the liquid state and break free from the surface.
  • Hence, elevated temperature results in a swifter rate of evaporation.
• Molecules can only break free from the intermolecular forces of attraction at the liquid's surface.
  • Consequently, a larger surface area facilitates a quicker evaporation rate.
• Air movement carries off recently evaporated water vapor, drying the air and enabling more water molecules to escape.
  • Therefore, boosting air movement (referred to as draughts indoors) enhances the rate of evaporation.

Evaporation & Cooling

• Evaporation has a cooling effect, making it useful for lowering temperatures.
  • When a substance evaporates, it absorbs heat from its surroundings, including nearby objects.
  • This cooling effect can be utilized in refrigeration and air conditioning systems.
  • Objects in contact with evaporating liquids experience cooling as the liquid transitions to a gas state.
  • Refrigerators and air conditioners exploit this phenomenon to maintain lower temperatures within their compartments.

Boiling vs Evaporation

• Boiling represents the transition of a substance from a liquid to a gas state.
  • Boiling occurs specifically at the boiling point of the substance.
  • During boiling, the change of state occurs uniformly throughout the liquid, often evidenced by the formation of bubbles, such as those observed in boiling water.