Evaporation (Cambridge (CIE) IGCSE Physics): Revision Note

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 when the particles with most energy leave

  • Therefore liquids are cooled down by evaporation

Factors affecting evaporation & explaining cooling

Extended Tier Only

Factors affecting evaporation

• These factors all affect the rate of evaporation

Temperature of the liquid 

• 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 of the liquid

• 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 replaces the air above the liquid with drier air, which accepts water vapour more easily

  • Therefore increasing air movement (e.g. wind or a fan) increases the rate of evaporation

Evaporation and cooling

• The process of evaporation can be used to cool things down

• As evaporation occurs, the liquid cools

  • This is because the most energetic particles escape from the surface

  • The average kinetic energy of the remaining particles reduces, so the temperature of the liquid falls

• An object in contact with an evaporating liquid is cooled

  • This is because thermal energy is transferred from the object to the cooler liquid

  • This process is used in some refrigerators and air conditioning units

Boiling vs evaporation

Extended Tier Only

• 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 throughout the liquid (seen as bubbles at the bottom of a pan of boiling water, for example)

• The following table shows the difference between boiling and evaporation: