Evaporation & Condensation (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

Evaporation & Condensation

Extension Tier only

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

When heated particles in a beaker of water gain kinetic energy they move around more. Arrows show this movement and that particles on the surface of the liquid can escape
Evaporation occurs when more energetic molecules near the surface of a liquid escape
  • 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

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