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extra advanced notes on gas laws, ideal and non-ideal gasesSTATES OF MATTER - properties of gases and liquids (fluids) and solids

9. Explaining evaporation and boiling (state changes liquid <=> gas/vapour)

Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry and also helpful for UK advanced level chemistry students aged ~16-18 and US grades 11-12 K12 honors.


Evaporation and Boiling (state change of liquid to gas)

Explained using the kinetic particle theory of gases and liquids

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  • 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.

gcse chemistry change in distribution of speeds kinetic energies with change in tempearture

  • 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.
  • 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.
  • particle model explaining evaporation from liquid surface to gas vapour higher speed kinetic energy molecules escape
  • 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).
  • 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.
  • 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

Learning objectives for the process of evaporation and boiling.

Be able to draw particle pictures to illustrate and explain boiling and evaporation.

Know what we mean by boiling and evaporation - the state change from liquid to gas or vapour (vapor)

Know the difference between boiling and evaporation.

Be able to use the kinetic particle theory of matter to explain boiling and evaporation

Know that particles in a liquid have a wide variety of speeds (velocities) and kinetic energies.

Know that highest kinetic energy molecules vaporise first.

Know that thermal energy is absorbed by the liquid as it evaporates or boils - endothermic processes.


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