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Thermal energy: 4.11 Using kinetic particle theory to
explain the factors
that affect the rate of evaporation of a liquid and rate of condensation of a
gas or vapour (vapor)
Doc Brown's Physics exam study revision notes
INDEX for my physics notes on particle model theory
explaining
state changes, latent heat, heating and cooling curves
4.11
Factors that affect
the rate of evaporation and condensation
(KE is shorthand for kinetic energy)
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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).
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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.
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Factors affecting the rate of
condensation
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The cooler the gas, the faster
it condenses - more lower KE particles can be attracted together.
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The lower the temperature of the
surface the gas is in contact with.
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The lower the airflow over the
surface, this keeps the concentration of the condensing gas as high as
possible.
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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.
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Because of this, being scalded by
steam is worse than be scalded by boiling hot water.
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Both involve transfer of thermal
energy due to the heat capacity of liquid water.
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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'.
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Evaporation is when the
highest kinetic energy particles of a liquid escape from the surface ie can
overcome the attractive forces of the bulk of particles.
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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.
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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.
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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.
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Factors affecting the rate of
evaporation
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The higher the liquid
temperature, the faster the rate of evaporation - more particles with enough
kinetic energy to escape from the surface (graph above).
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Reminder of particle model of evaporation
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The greater the surface area,
the faster the evaporation - more area, more chance of evaporation.
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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.
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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
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 ale to describe that factors that affect the rate of condensation of
a gas/vapour to a liquid.
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INDEX of notes on Particle model theory
state changes and latent heat
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