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STATES OF MATTER -
properties of gases and liquids (fluids) and solids
10. Explaining condensation
using the kinetic particle model theory and how boiling and condensation explain
the process of distillation (state changes gas/vapour <=>
liquid)
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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
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IGCSE/O level chemistry courses, ~US grades 9-10 chemistry notes [page updated
Nov 29th 2025]
INDEX of all my notes on the states of matter
GCSE (~US grades 8-10) level multiple choice QUIZ on
the states of matter: gases, liquids & solids
10a. Condensing (gas to liquid)
– the process of condensation
Explained using the kinetic particle theory of gases and
liquids
- On cooling, gas particles lose kinetic energy,
they slow down and eventually become attracted together via intermolecular
forces to form a liquid
i.e. they haven't enough kinetic energy to remain free in the gaseous state.
- There is an increase in order as the particles are much closer together and can form clumps of molecules.
- The lowest kinetic energy molecules are most likely to condense first.
The process requires thermal
energy (heat energy) to be lost to the surroundings i.e. heat given out, so
condensation is exothermic (ΔH –ve in advanced level
notation).
- This is why steam has such a scalding
effect, its not just hot, but you get extra heat transfer to your skin
due to the exothermic condensation on your surface!
In your home you see condensation on cold windows and steam is
invisible, and what you refer to as steam coming out of a kettle is
actually a cloud of water droplets from the condensation of steam vapour
in the cooler air.
Factors affecting the rate of condensation of a gas–vapour
- The lower the temperature of the gas the faster it condenses because the
particles on average have less kinetic energy to overcome the attractive
intermolecular forces i.e. they gas particles are more likely to aggregate
into drops of liquid.
- The colder the surface the gas condenses on, the faster the heat
transfer to reduce the kinetic energy of the gas particles, so the faster
the gas/vapour can condense.
- The higher the concentration of vapour in air, the faster condensation
can take place. The particles are closer together and more chance of
colliding and combining to form liquid droplets.
10b. Distillation
– the process of distilling a liquid
This page also provides links to
other methods of
separation - described and explained.
The process of distillation involves boiling (liquid ==>
gas/vapor) and the reverse process of condensation (gas/vapour ==> liquid)
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
For revision that are aligned to the major exam boards
(WJEC, CCEA, CIE IGCSE, AQA, Edexcel, OCR Gateway, OCR 21st Century).
They are built this around the syllabus specifications
and included examples, exam tips, and misconceptions to
support student preparation.
Using the Kinetic
Particle Model: Condensation & Distillation
Condensation of
vapour/gas
- Definition: Change of state from gas → liquid.
- Explanation (kinetic particle model):
- Gas particles have high kinetic energy and move freely.
- Cooling reduces kinetic energy → particles slow down.
- Intermolecular forces pull particles closer together → liquid forms.
- Examples:
- Water vapour condensing on a cold window.
- Steam turning back into water droplets.
Distillation of
liquids
- Definition: Separation technique using differences in
boiling points.
- Process (kinetic particle theory):
- Heating: Liquid mixture is heated → particles gain
kinetic energy.
- Boiling: The component with the lower boiling point
vaporises first.
- Evaporation: Gas particles escape into condenser.
- Cooling: Gas particles lose kinetic energy in
condenser → condensation occurs.
- Collection: Condensed liquid (distillate) is
collected separately.
- Examples:
- Distilling water from salt solution (simple distillation).
- Separating ethanol from water (fractional distillation).
Typical Exam Board
Requirements
about condensation and distillation
- Important to describe clearly particle explanations and diagrams of
distillation apparatus.
- Link kinetic theory and practical separation methods (simple versus
fractional distillation).
- Link condensation to water cycle and distillation to purification.
- Includes crystallisation, evaporation, and distillation in separation
techniques.
- Describe particle motion and energy transfer in condensation.
- Apply distillation to real-world contexts (e.g., crude oil refining,
purifying water).
Student Exam Tips
about condensation and distillation
- Draw apparatus diagrams: Label condenser, flask,
thermometer, and collection vessel.
- Always mention energy changes: Condensation = loss of
kinetic energy; distillation = gain then loss.
- Link to real-world examples: Water cycle (evaporation +
condensation), crude oil fractional distillation.
- Use precise terms: “Intermolecular forces” not “bonds”
when describing condensation.
- State boiling points: Distillation relies on
differences in boiling points.
Common
Misconceptions
about condensation and distillation
- “Condensation is the same as freezing” → Freezing is
liquid → solid; condensation is gas → liquid.
- “Distillation creates new substances” → It only
separates; no chemical change occurs.
- “All liquids boil at 100 °C” → Only pure water at 1 atm
does; boiling points vary.
- “Condensation happens because particles stick together
randomly” → It’s due to reduced kinetic energy and intermolecular
forces.
- “Fractional distillation works for solids” → It separates
liquids with different boiling points, not solids.
Quick Recap about
condensation and distillation
- Condensation: Gas → liquid, cooling reduces kinetic
energy, particles come closer.
- Distillation: Separation by boiling and condensation,
relies on differences in boiling points.
- Kinetic particle model link: Heating increases kinetic
energy (distillation), cooling decreases kinetic energy (condensation).
Learning objectives
for the state change of condensation
Know what we mean by condensation -
the sate change from gas/vapour to liquid.
Be able to use the kinetic particle
theory of matter to explain the state of matter change of
condensation
Know that particles in a gas/vapor
have a wide variety of speeds (velocities) and kinetic energies.
Know that lowest kinetic energy
molecules condense first.
Know that thermal energy is
released by the gas as it condenses - exothermic processes.
Know that distillation involves
boiling followed by condensation and the state changes are liquid
==> gas/vapour ==> liquid
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Based on the syllabus-specifications for students taking the IGCSE/GCSE
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explaining condensation using the kinetic particle theory and how
condensation is part of the process called distillation,
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INDEX of all my notes on the states of matter
GCSE (~US grades 8-10) level multiple choice QUIZ on
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