Evaporation (CIE IGCSE Physics)

• Evaporation is a change in state of a liquid to a gas
• It happens; 
  • At any temperature
  • Only from the surface of a liquid

 

• The molecules in a liquid have a range of energies
  • Some have lots of energy, others have very little
  • Their average energy relates to the temperature of the liquid

    

• Evaporation occurs when more energetic molecules moving near the surface of the liquid have enough energy to escape
  • The average energy of the liquid is reduced
  • Therefore liquids are cooled down by evaporation

Evaporation occurs when more energetic molecules near the surface of a liquid escape

 

EXTENDED

Temperature, Surface Area & Air Movements

These factors all affect the rate of evaporation   

 

• Increased temperature increases the kinetic energy of the molecules in the liquid
  • Molecules with more energy are more likely to overcome the intermolecular forces holding them in the liquid state and escape the surface
  • Therefore higher temperature leads to a higher rate of evaporation

• Molecules only escape the intermolecular forces of attraction at the surface of the liquid
  • Therefore a larger surface area leads to a higher rate of evaporation
• Air movement carries away the water vapour which has just evaporated
  • This dries the air and allows more water molecules to escape 
  • Therefore increasing air movement (when indoors this is sometimes called draughts) increases the rate of evaporation

EXTENDED

   

Evaporation & Cooling

• The process of evaporation can be used to cool things down:
  • If an object is in contact with an evaporating liquid, as the liquid cools the solid will cool as well
  • This process is used in refrigerators and air conditioning units

EXTENDED

• Boiling is also a change in state from liquid to gas
  • Boiling happens only at the boiling point of the liquid
  • The change of state happens all through the liquid (seen as bubbles in boiling water, for example)