Evaporation (Cambridge (CIE) IGCSE Physics)

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

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

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 accept water vapour more easily

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

Evaporation & 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 leave, reducing average kinetic energy

• Placing an object next to this liquid cools the object

  • This is because the cooler liquid absorbs thermal energy from the object

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

Boiling vs evaporation

Extended tier only

Difference between evaporation and boiling

• 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)

Table showing the difference between evaporation and boiling