STATES OF MATTER - properties of gases and liquids (fluids) and solids

9. Using the kinetic particle model theory to explain evaporation and boiling and the difference between them (state change liquid ==> gas/vapour)

[Author © Dr WP Brown PhD: Doc Brown's chemistry exam revision notes on states of matter - physical properties of gases, liquids and solids, suitable for students of UK GCSE level and international IGCSE/O level chemistry courses, ~US grades 9-10 chemistry notes [page updated Nov 29th 2025]

GCSE (~US grades 8-10) level multiple choice QUIZ on the states of matter: gases, liquids & solids

• Evaporation is when particles of a liquid escape to form a gas or vapour i.e. water evaporating into the air.
• Because of random collisions, the particles in a liquid have a variety of speeds and kinetic energies. On heating, particles gain kinetic energy and move faster and are more able to overcome the intermolecular forces between the molecules i.e. some particles will have enough kinetic energy to overcome the attractive forces holding the particles together in the bulk liquid.
  • Even without further heating, evaporation occurs all the time from volatile liquids, but it is still the higher kinetic energy particles that can overcome the attractive forces between the molecules in the bulk of the liquid and escape from the surface into the surrounding air.
• In evaporation and boiling (both are vaporisation) it is the highest kinetic energy molecules that can ‘escape’ from the attractive forces of the other liquid particles.
  • The particles lose any order and become completely free to form a gas or vapour.
  • Also, because the highest kinetic energy particles have escaped, the liquid is cooler, because the lower kinetic energy particles are left.
  • This is equivalent to energy being used to evaporate a liquid (see below).
  • When a liquid evaporates or boils the particles absorb thermal energy, so the process is endothermic.

• The graph above shows how the distribution of kinetic energy and speed of particles changes with changes in temperature - with increase in temperature, the average speed and kinetic energy of the particles increases.
  • The graph for the distribution of particle kinetic energies is similar.
  • This is usually dealt with advanced level chemistry, but it does help explain why a liquid doesn't suddenly all completely vapourise!

• Note that the random movement and collisions of the particles creates a wide range of speeds and kinetic energies.

• When the temperature is increased, more particles have a greater kinetic energy and greater speed, but only the highest speed/kinetic energy particles can escape from the surface (only the very right-hand section of the graph curves)
• Below is a particle model of evaporation.
• 

• Energy is needed to overcome the attractive forces between particles in the liquid and is taken in from the surroundings.
  • In boiling, heat energy must be continually supplied e.g. from an electrical heating element or Bunsen burner etc.
  • In the case of evaporation, the heat is taken from the liquid, so an evaporating liquid cools - the lower speed/kinetic energy particles are left behind.
• This means heat is taken in, so evaporation and boiling are endothermic processes (ΔH +ve)
• The energy required for the state change of liquid to gas/vapour is called the latent heat of vaporisation (vaporisation).
• See section (c) for more on factors affecting the rate of evaporation of a liquid

• If the temperature is high enough boiling takes place and bubbles of gas form in the bulk liquid – something you don't see in evaporation, because that can only occur on the surface of a liquid.
• Boiling is rapid vapourisation anywhere in the bulk liquid and at a fixed temperature called the boiling point and requires continuous addition of heat.
  • In boiling, bubbles of gas/vapour form in the bulk of the liquid, not so with evaporation, which is a surface effect at temperatures below the boiling point.
  • Boiling point depends on the ambient pressure, the lower the gas pressure above the liquid, the lower the boiling point of the liquid.
  • This is why tea brewed on the top of high mountain isn't quite as good as at sea level, the water boils at a lower temperature and doesn't extract substances from the tea leaves as efficiently!
  • In the past, measuring the boiling point of water was used to estimate the height of land above sea level!
• The rate of boiling is limited by the rate of heat transfer into the liquid.
• See also that Boiling is the first stage in distillation (section 10.)

• Evaporation takes place more slowly than boiling at any temperature between the melting point and boiling point, and only from the surface, and results in the liquid becoming cooler due to loss of higher kinetic energy particles.
• Factors affecting the rate of evaporation of a liquid.
  • The higher the temperature of the liquid, the faster it evaporates, because more particles have sufficient kinetic energy to overcome the intermolecular forces of the bulk liquid and can escape from the liquid surface.
  • The larger the surface area of given volume of liquid, the faster it evaporates, because there is a greater probability of particles escaping.
  • The greater the airflow over a liquid the faster it evaporates because its stops a build–up of vapour particles which may hit the surface and condense! The airflow lowers the concentration of evaporated particles by sweeping them away and so more readily replaced by freshly evaporated particles.
  • Please note that the best conditions for drying washing are a warm sunny day, a good breeze, and spreading the clothes out as much as possible to increase their surface area (I get told off about this one!).
• Energy changes for these physical changes of state for a range of substances are dealt with in a section of the Energetics Notes and also here in section 14.
• 14. Comparison of latent heat changes in physical changes of state for different substances

Key points about evaporation and boiling

 Designed for the major UK and international exam boards (WJEC, CCEA, CIE IGCSE, AQA, Edexcel, OCR Gateway, OCR 21st Century).

The content is suitable for the syllabus specifications and added examples, exam tips, and misconceptions to help students prepare effectively.

Kinetic Particle Model: Evaporation versus Boiling

Core Principles

• Kinetic Particle Model: All matter is made of particles (atoms/molecules/ions) in constant motion.
• Energy and Phase Change: Heating increases kinetic energy → particles move faster → intermolecular forces are overcome → phase changes occur.

Evaporation of a liquid

• Definition: The process where particles at the surface of a liquid escape into the gas phase.
• Conditions: Occurs below the boiling point at any temperature.
• Explanation (kinetic particle theory):
  • Particles at the surface have a range of kinetic energies.
  • Some particles gain enough energy (from collisions) to overcome intermolecular forces.
  • They escape into the air as gas molecules.
• Example: A puddle of water disappearing on a warm day, even if the temperature is only 20 °C.

Boiling liquid

• Definition: Rapid vaporisation that occurs when the vapour pressure equals atmospheric pressure.
• Conditions: Happens at a fixed temperature (boiling point) for a given pressure.
• Explanation (kinetic particle theory):
  • Heating increases kinetic energy of all particles.
  • At boiling point, particles throughout the liquid (not just at the surface) have enough energy to break intermolecular forces.
  • Bubbles of vapour form inside the liquid and rise to the surface.
• Example: Water boiling at 100 °C at 1 atm pressure.

Key Differences  between evaporation and boiling

Typical Exam Board Requirements

• Emphasis on particle theory, energy changes, and everyday examples (drying clothes versus boiling kettle).
• Focus on distinguishing evaporation and boiling with particle diagrams.
• Clear explanation using kinetic theory and linking to pressure/temperature conditions.
• Questions often asks for differences in terms of particle energy and location (surface versus bulk).
• Relate to practical contexts (distillation, evaporation in crystallisation).
• Discuss in terms of particle motion and energy transfer.
• Aware of applied contexts (domestic-everyday life and industrial processes).

Student Exam Tips for evaporation and boiling

• Use particle diagrams: Draw particles at the surface escaping (evaporation) versus bubbles forming throughout (boiling).
• Mention energy distribution: Evaporation relies on particles with higher-than-average kinetic energy.
• Always state conditions: Boiling occurs at a fixed temperature; evaporation does not.
• Link to pressure: Boiling point changes with atmospheric pressure (e.g., lower on mountains).
• Contextual examples: Drying clothes (evaporation), kettle boiling (boiling).

Common Misconceptions about evaporation and boiling

•  “Evaporation only happens at boiling point” → Wrong. It happens at any temperature.
•  “All particles escape during evaporation” → Only higher-energy surface particles escape.
•  “Boiling and evaporation are the same” → They differ in conditions, speed, and particle involvement.
•  “Boiling point is fixed everywhere” → It varies with atmospheric pressure.
•  “Evaporation requires bubbles” → Bubbles form only during boiling.

Quick Recap (Exam-Style) of evaporation and boiling

• Evaporation: surface, slow, any temp, higher-energy particles escape.
• Boiling: bulk, rapid, fixed temp, bubbles form when vapour pressure = atmospheric pressure.
• Both explained by kinetic particle model: heating → increased kinetic energy → particles overcome intermolecular forces.

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