Evaporation & Condensation (Oxford AQA IGCSE Physics)

Evaporation & Condensation

The cooling effect of evaporation

• Energy can be transferred by evaporation

• Evaporation is a change in the state of a liquid to a gas

  • It has a cooling effect

• Evaporation is different to boiling or vaporisation

  • Evaporation happens at any temperature, whereas boiling only occurs at the boiling point of the substance

  • Evaporation only occurs to particles at the surface of the liquid, whereas every particle changes state when a liquid is vaporised

• The molecules in a liquid have a range of energies

  • Some have lots of kinetic energy, others have very little

  • Their average kinetic 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 overcome the intermolecular forces of attraction holding the molecules in their liquid state

  • The average energy of the liquid is reduced

  • Therefore liquids are cooled down by evaporation

Evaporation from the surface of a liquid

• 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

Factors affecting 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 their liquid state and escape the surface

  • Therefore higher temperatures lead 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