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4.11 Factors that affect the rate of evaporation and condensation

(KE is shorthand for kinetic energy)

• Condensation occurs when a gas/vapour is cooled sufficiently to a low enough temperature to allow the attractive forces to be strong enough to attract the particles together as a liquid. This can only happen if the kinetic energy of the particles is low enough (the lower the temperature the smaller the kinetic energy).

  • Water vapour in the air condenses out on cold surfaces in the winter eg window condensation, invisible steam from a boiling kettle condenses out into clouds of tiny droplets of water, which technically isn't steam! and rain drops form in the higher cooler regions of the atmosphere.

  • Factors affecting the rate of condensation

    • The cooler the gas, the faster it condenses - more lower KE particles can be attracted together.

    • The lower the temperature of the surface the gas is in contact with.

    • The lower the airflow over the surface, this keeps the concentration of the condensing gas as high as possible.

  • When a vapour/gas is condensed the latent heat of vaporisation must be removed to cool the particles down sufficiently for condensation to take place.

    • Because of this, being scalded by steam is worse than be scalded by boiling hot water.

    • Both involve transfer of thermal energy due to the heat capacity of liquid water.

    • BUT, water vapour must be first condensed, so initially you are scalded by the release of the latent heat of vaporisation = the 'latent heat of condensation'.

• Evaporation is when the highest kinetic energy particles of a liquid escape from the surface i.e. can overcome the attractive forces of the bulk of particles.

  • The greater the KE of a liquid surface particle, the greater the chance to escape and become a gas particle. Evaporation can take place at any temperature between a substance's melting point and boiling point.

  • As the highest KE particles escape, leaving the slower lower KE particles, the bulk of the liquid will cool, so a cooling effect accompanies the evaporation of a liquid.

  • The cooling effect of sweating is due to evaporation of water from your skin and the thermal energy absorbed from your body in the process.

• Factors affecting the rate of evaporation

  • 

  • The higher the liquid temperature, the faster the rate of evaporation - more particles with enough kinetic energy to escape from the surface (graph above).

  • 

  • Reminder of particle model of evaporation

  • The greater the surface area, the faster the evaporation - more area, more chance of evaporation.

  • The greater the airflow over the surface of the faster the evaporation rate - the air can become saturated with the vapour of the liquid, so it is more readily replaced if the already evaporated liquid is swept away by air flowing over the surface.

    • Efficient drying of washing is a good example of these three factors - you need:

      • a warm sunny day (higher temperature) to increase the KE of the water molecules,

      • the washing well spread out on the line to surface the evaporation area

      • and a nice breeze sweeping the evaporated water away!

  • When water evaporates the latent heat of vaporisation is absorbed by the water molecules giving a cooling effect.

    This is why sweating cools your body, especially if you in a cooling breeze on a hot summer's day.

INDEX of notes on Particle model theory state changes and latent heat

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Key points Thermal energy & particle models

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A structured and exam-board-inclusive set of summary revision notes on Factors Affecting the Rate of Evaporation, explained using particle theory, tailored for GCSE/IGCSE Physics students across WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

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What Is Evaporation?

• Evaporation is the change of state from liquid to gas occurring only at the surface of a liquid.
• It happens below the boiling point when high-energy particles escape from the surface.
• Particle theory explains this as particles with enough kinetic energy overcoming intermolecular forces.

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Particle Theory Explanation of evaporation

• Liquids contain particles with a range of kinetic energies.
• Faster particles near the surface may escape into the air as gas.
• As these particles leave, the average energy of remaining particles decreases, causing cooling.

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Factors Affecting Rate of Evaporation

Factor	Effect	Particle Theory Explanation
Temperature ↑	Evaporation ↑	Particles gain more kinetic energy → more can escape
Surface Area ↑	Evaporation ↑	More particles exposed at surface → more escape opportunities
Air Movement ↑	Evaporation ↑	Removes saturated air → prevents re-condensation
Humidity ↓	Evaporation ↑	Drier air allows more particles to escape without returning
Type of Liquid	Varies	Liquids with weaker intermolecular forces evaporate faster

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Required Practicals (varies by board) on evaporation

• Investigate evaporation using different surface areas, temperatures, and airflow.
• Use mass loss over time to measure evaporation rate.
• Observe cooling effect using thermometers and wet cloths.

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Exam Tips about evaporation

• Always link energy transfer to particle motion.
• Use particle diagrams to show surface escape.
• Know the difference between boiling and evaporation:
  • Boiling: throughout liquid, at boiling point.
  • Evaporation: surface only, below boiling point.
• Use correct terminology: kinetic energy, intermolecular forces, surface particles.

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Common Misconceptions about evaporation

• Evaporation only happens at boiling point → It occurs below boiling point.
• All particles evaporate at once → Only high-energy surface particles escape.
• Evaporation doesn’t cause cooling → It does, due to loss of high-energy particles.
• Humidity has no effect → High humidity slows evaporation.

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Keywords, phrases and learning objectives for particle models and thermal energy

Be able to use the kinetic particle theory to explain factors affecting rate of evaporation i.e. change of liquid state to a gas or vapour/vapor.

Be able to describe that factors that affect the rate of condensation of a gas/vapour to a liquid.

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Revision notes on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid based on the syllabus-specifications for students taking IGCSE/GCSE level physics examinations, summary revision notes and key points on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid for students taking the AQA igcse/gcse physics notes on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid, Edexcel gcse physics notes on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid,  OCR 21st century GCSE physics notes on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid, OCR gateway GCSE physics notes on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid, WJEC gcse physics notes on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid, CCEA gcse physics notes on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid for students taking CIE Cambridge igcse physics, exam revision notes on using kinetic particle theory to explain the factors controlling the rate of evaporation of a liquid, useful for US grade 9-10 physics courses, importance of using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour in GCSE level physics, What you need to know about using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour for GCSE level physics, Explaining the use of using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour knowledge in GCSE level physics, Examples of using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour explained when studying GCSE level physics, What is significant about using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour, describing the theory of using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour when studying GCSE level physics, revision notes for using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour in exams, online exam help for using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour, revision notes about using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour, what do I need to learn about using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour for by GCSE physics exam? help to understand the using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour topic in preparation for GCSE physics exam question, how to prepare for questions involving using kinetic particle theory to explain the factors controlling the rate of condensation of a gas/vapour in a GCSE physics examination?

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INDEX of notes on Particle model theory state changes and latent heat