Evaporation (Cambridge O Level Physics)

• 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)

Comparing Evaporation and Boiling

	Evaporation	Boiling
Change of state?	Liquid to gas	Liquid to gas
Temperature at which this occurs?	At any temperature	Only at boiling point
From where in the liquid?	From the surface	Throughout the liquid

• 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

Process of Evaporation

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

Factors affecting the Rate of Evaporation

• The factors that affect the rate of evaporation are
  • Temperature
  • Surface area
  • Air movement

Temperature

• 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

Surface Area

• 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

• 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

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
  • This is also why sweating cools skin down - the water evaporates using energy from the body's thermal store