Evaporation (CIE IGCSE Physics)

• 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

 

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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

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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

	Evaporation	Boiling
Change of state?	Liquid to gas	Liquid to gas
Temperature?	Any temperature between melting and boiling point	Boiling point
Location in liquid?	From the surface	Throughout the whole liquid