Evaporation (Cambridge (CIE) O Level Physics): Revision Note

Boiling vs Evaporation

• 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

Cooling by Evaporation

• 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