|
STATES OF MATTER -
properties of gases and liquids (fluids) and solids
3.
Simple kinetic particle model of
gases
to explain their physical properties including gas pressure & effect of temperature
change
[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 28th 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
 3.
The particle model of a GAS
- WHAT IS THE
GASEOUS STATE OF MATTER?
- WHAT ARE THE
PROPERTIES OF A GAS?
- HOW DO GASEOUS
PARTICLES BEHAVE?
- How does the
kinetic particle theory of gases explain the properties of gases?
- A gas has no fixed shape or volume, but always spreads out
to fill any container - the gas molecules will diffuse into any space
available.
- There are almost no forces of attraction
between gas particles
so they are completely free of each other and spread out to completely fill a
container - hence their very low density.
- The particles are widely spaced and scattered and
always moving rapidly at random throughout the container so there is no order in the system.
- The particles move
linearly and rapidly in all directions,
and frequently collide with each other and the side of the container.
-
The collision of gas particles with the surface of a container causes gas
pressure, on bouncing off a surface they exert a force in doing so.
With increase in temperature, the particles move faster as they gain kinetic
energy, the rate of collisions between the particles themselves and the
container surface increases and this increases gas pressure eg in a steam
locomotive or the volume of the container if it can expand eg like a balloon.
Using the particle model to explain the properties of a Gas
Gases have a very low density
(‘light’) because the particles are so spaced out in the container (density = mass
/ volume) - the particles are unrestricted and spread out to fill any
container.
- The particle intermolecular forces are very weak and their kinetic energy is
high enough to prevent then condensing to the considerably more dense state
of liquid.
Density order: solid > liquid
>>> gases
Gases flow freely because there are no effective forces of attraction between the
gaseous particles – molecules.
- Ease of flow order: gases >
liquids >>> solids (no real flow in solid unless you finely powder
it!)
- Because of this gases and liquids are described as
fluids.
Gases have no surface, and no fixed shape or
volume, and because of lack of particle attraction, they
always spread out and fill any container (so gas volume = container volume).
Gases are readily compressed because of the ‘empty’ space between the particles.
- Ease of compression order: gases
>>> liquids > solids (almost impossible to compress a
solid)
Gas pressure
- When a gas is confined in a container the particles will cause and exert a
gas pressure which is measured in atmospheres (atm) or Pascals (1.0
Pa = 1.0 N/m2), pressure is force/area i.e. the effect of all
the collisions on the surface of the container.
- All particles have mass and their movement gives them kinetic energy
and momentum.
- The gas pressure is caused by the force created by millions of impacts of
the tiny individual gas particles on the sides of a container.
- For example – if the number of gaseous particles in a container is doubled, the gas
pressure is doubled because doubling the number of molecules doubles the
number of impacts (collisions) on the side of the container so the total impact force per
unit area is also doubled.
- This doubling of the particle impacts doubling the pressure is pictured in diagrams below.
|
|
2 x particles
===>
P x 2 |
 |
-
Effect of temperature change
- If the volume of a sealed container is kept constant and the gas inside
is heated to a higher temperature, the gas pressure increases.
- The reason for this is that as the particles are heated they gain kinetic
energy and on average move faster.
- Therefore they will collide with the sides of the container with a
greater force of impact, so increasing the pressure.
- There is also a greater frequency of collision with the sides of the
container BUT this is a minor factor compared to the effect of increased kinetic
energy and the increase in the average force of impact.
- Therefore a fixed amount of gas in a sealed container of constant volume,
the higher the temperature the greater the pressure and the lower the
temperature the lesser the pressure.
-
See Part 19.
Particle model - gas pressure-volume-temperature calculations
See Part 20. The
Combined
gas law equation - more complicated calculations involving P-V-T
- If the ‘container’ volume can change, gases readily expand* on heating because of the lack of particle attraction, and
readily contract on cooling.
- On heating, gas particles gain kinetic energy,
move faster and hit the sides of the container more frequently, and
significantly, they hit with a greater force.
- Depending on the container situation, either or both of the pressure or volume will increase (reverse on cooling).
- Note:
*
It is the gas volume that expands NOT the molecules, they stay the same
size!
- If there is no volume restriction
the expansion on heating is much greater for gases than liquids or solids
because there is no significant attraction between gaseous particles. The
increased average kinetic energy will make the gas pressure rise and so
the gas will try to expand in volume if allowed to e.g. balloons in a warm
room are significantly bigger than the same balloon in a cold room!
- Thermal energy (heat
energy) conduction in gases
- All gases are very poor conductors of thermal energy, energy which is due to the
kinetic energy of the moving particles.
- Heat energy is transferred by 'hotter' higher kinetic energy gas particles
colliding with 'cooler' lower kinetic energy particles so raising their kinetic
energy and spreading the heat energy.
- However, the density of gases is very low, so the density or rate of
'collision transfer' is quite low.
- Therefore gases are very good insulators e.g. think of their used in house
insulation where air is trapped in various ways like foam or fibre glass loft
insulation.
- Electrical conduction in gases
- Electrical conduction requires the presence of free IONS or free ELECTRONS
i.e. particles that can carry an electrical charge.
- Gases are poor conductors of electricity because they are usually not in an
ionic or ionised form.
- However, applying a very high potential difference of thousands of volts,
especially with a low gas pressure, can cause the formation of free ions and
electrons and electrical conduction can happen.
- Strip lighting and neon signs use this effect.
- Diffusion in gases
-
Brownian motion in gases
- When smoke particles suspended in air are viewed under a
microscope they appear to 'dance around' when illuminated with a light
beam at 90o to the viewing direction.
- This is because the
pollen grains show up by reflected light and 'dance' due to the
millions of random hits from the fast moving air molecules.
- This phenomenon is
called 'Brownian motion' after a
botanist called Brown first described the effect (see
Part 5 for evidence for Brownian motion in
gases).
- At any given instant of time, the
particle hits will not be even all round the surface of the smoke particles, so
they get a greater number of hits in a random direction and then another, hence
the smoke particles zig-zag around in all directions with random movement.
Learning objectives
for using the particle model of a gas to explain its properties
Image adapted from Wikipedia, but in reality the 'red and blue' particles are
moving much faster (e.g. 200 m/s) and colliding millions of time a second.
Be able to describe the kinetic particle model of a gas.
Be able to draw particle pictures to
illustrate and explain the physical properties of gases.
Know, and why, for the same substance, gases have a much lower density
than liquids or solids.
Know and explain that gases have no shape or fixed volume - the particles
move and spread out to fill any container.
Know and explain why gases are much more easily compressed to a smaller
volume compared to solids and liquids because of the relatively large space
between gas particles.
Be able to explain pressure in terms of the kinetic particle model of a
gas i.e. the collective effect of many impacts of gas molecules on the
enclosing surface..
Know and explain that very weak forces between gas particles allows a gas
to be a fluid and flow freely e.g. under pressure in a gas pipe.
Explain using the particle model that increasing the number of gas
particles in a container, increases the number of collisions of gas
particles with the surface and so increases pressure.
Be able to explain the effect of changing temperature in terms of the
kinetic particle model of a gas e.g. increasing temperature can increase
volume or pressure depending on conditions and explain this is due to the
increase in the average kinetic energy of the particles.
Know that the thermal energy, the internal energy of a gas is the kinetic
energy of the particles.
Know that gas particles are not usually electrically charged, gases are
very poor conductors of electricity (but a high p.d. may produce ions and
electrical conduction).
Be able to describe and explain microscope observations of Brownian motion in
gases e.g. the motion of suspended smoke particles in air.
A summary
revision guide on the Kinetic Particle Theory of
Gases,
tailored for GCSE/IGCSE chemistry students across exam boards (WJEC,
CCEA, CIE, AQA, Edexcel, OCR, and equivalent US Grade 9–10 courses).
Kinetic Particle
Theory of Gases
Key Ideas
- Particles in gases:
- Far apart compared to solids/liquids.
- Move randomly and rapidly in all directions.
- Weak/no forces between particles (except during collisions).
- Collisions:
- Elastic (no loss of kinetic energy overall).
- Cause observable effects like pressure.
Cause of Gas
Pressure
- Definition: Pressure is the force exerted per
unit area.
- Origin in gases:
- Gas particles collide with the walls of the container.
- Each collision exerts a tiny force.
- The total force from many collisions per
unit area = gas pressure.
- Factors affecting pressure:
- More frequent collisions → higher pressure.
- Stronger collisions (higher particle speed) → higher
pressure.
Effect of
Temperature
- Temperature and kinetic energy:
- Temperature is a measure of the average kinetic
energy of particles.
- Higher temperature → particles move faster.
- Consequences:
- Faster particles collide with container walls more often and
with greater force.
- Pressure increases if volume is fixed.
- Practical links:
- Heating a sealed container increases pressure (risk of
explosion).
- Cooling reduces particle speed → lower pressure.
Typical Exam Board
Requirements
|
Specification Emphasis |
| Kinetic theory, pressure-temperature
relationship, gas laws context. |
| Cause of pressure, effect of
temperature, particle motion. |
| Particle theory, pressure from
collisions, temperature effects. |
| Gas particle motion, pressure
definition, temperature link to kinetic energy. |
Common
Misconceptions
- Pressure is caused by particles "pushing" each other →
Incorrect.
Pressure is due to collisions with container walls.
- Temperature only affects pressure if particles "expand" →
Incorrect.
Temperature increases particle speed and collision force.
- Particles stop moving at low temperature →
Incorrect.
They slow down, but only stop at absolute zero (0 K).
Exam Tips
- Use keywords: random motion, collisions,
kinetic energy, pressure, temperature.
- Link cause → effect clearly: "Higher
temperature → faster particles → more frequent/forceful collisions →
higher pressure."
- Diagrams of particles in a container at different temperatures
often appear in exams—be ready to annotate them.
- Remember: Pressure proportional to collision frequency ×
collision force (qualitative, not quantitative at GCSE
level).
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UK GCSE level (~US grade 8-10) school chemistry revision
notes
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GCSE level and advanced pre-university level
revision notes. Detailed
notes on the states of matter and their properties.
Based on the syllabus-specifications for students taking the IGCSE/GCSE
level physics examinations summary revision notes and key points about the
kinetic particle theory for a gas - pressure and effect of
temperature,
for students taking the WJEC gcse
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exams e.g. US grade 9-10 physics courses
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 |
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