STATES OF MATTER -
properties of gases and liquids (fluids) and solids
A simple kinetic particle model of a liquid
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.
The particle model of a LIQUID
WHAT IS THE LIQUID
STATE OF MATTER?
WHAT ARE THE
PROPERTIES OF A LIQUID?
HOW DO LIQUID
How does the
kinetic particle theory of liquids explain the properties of liquids?
A liquid has a fixed volume at a given temperature but
its shape is that of the container which holds the liquid.
There are much greater forces of attraction between the particles in a liquid compared to
gases, but not quite as much as in solids and the particles are
sufficiently free to move past each other.
The particles are quite close together but still arranged at random throughout the container
due to their constant random movement, there is a little close range order as you can get clumps of particles clinging together temporarily
(as in the diagram above).
As well as moving rapidly in all directions, they
collide more frequently with each other than in gases due to
shorter distances between particles – much greater density - particles closer
With increase in temperature, the particles
move faster as they gain kinetic energy, so increased
collision rates, increased collision energy, increased rates of particle diffusion,
expansion leading to decrease in density.
If there were no intermolecular forces, liquids could not exist!
Using the particle model to explain the properties of a Liquid
Most liquids are just a little less dense than when they are
Liquids have a much greater density than gases (‘heavier’) because the particles are much closer
together because of the attractive forces.
Liquids usually flow freely
despite the forces of attraction between the particles but liquids are not as ‘fluid’ as gases.
- Water is a curious exception to this general rule, which is
why ice floats on water.
Liquids have a surface, and a fixed volume (at a particular temperature) because of the increased particle attraction, but the shape is not fixed and is merely that of the container itself.
- Note 'sticky' or viscous liquids have much stronger attractive forces between
the molecules BUT not strong enough to form a solid.
Liquids are not readily compressed because
there is so little ‘empty’ space between the particles, so increase in pressure
has only a tiny effect on the volume of a solid, and you need a huge increase in
pressure to see any real contraction in the volume of a liquid.
Liquids will expand on heating but nothing like as much as gases,
but more than solids, because of the greater particle attraction restricting the
expansion (will contract on cooling).
- Liquids seem to have a very weak 'skin' surface effect which is caused by
the bulk molecules attracting the surface molecules disproportionately.
- The expansion of a liquid is due to the higher average
kinetic energy of the particles and the more energetic collisions cause the
expansion. BUT, they are still held together by the intermolecular forces, which
restricts the expansion - this is not part of the kinetic particle theory!
- Note: When heated, the liquid particles gain kinetic energy and hit the sides of the container more frequently, and more significantly, they hit with a greater force, so in a
sealed container of liquid the pressure produced
in a liquid can be considerable!
- Thermal energy (heat
energy) conduction in liquids
- Most liquids are 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 liquid particles
colliding with 'cooler' lower kinetic energy particles so raising their kinetic
energy and spreading the heat energy.
- However, the density of liquids is much greater than gases (particles much
closer together), so the density or rate of
'collision transfer' is much higher, so liquids are better heat conductors than
- Liquid metals are very good heat conductors because of the freely moving
electrons that can carry the kinetic energy rapidly through the liquid. For more
details see 'metal structure'.
- Electrical conduction in liquids
- Electrical conduction requires the presence of free IONS or free ELECTRONS
i.e. particles that can carry an electrical charge.
- Most liquids are poor conductors of electricity (good insulators), but there
are important exceptions.
- For example, if a liquid contains ions e.g. salt solutions, then electrical
conduction can take place
- Liquid metals are very good electrical conductors because of the freely
moving electrons that can carry the electrical current rapidly through the
- For more details see 'electrolysis'
and 'metal structure'.
motion in liquids
- When pollen grains suspended in water 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 water molecules.
- This phenomenon is
called 'Brownian motion' after a
botanist called Brown first described the effect (see
Brownian motion in gases).
- At any given instant of time,
the particle hits will not be even all round the surface of the pollen grains, so they get a greater
number of hits in a random direction and then another, hence the pollen grains zig-zag around
in all directions at random.
particle model of liquids and explaining their properties
Be able to describe the kinetic particle model of a liquid.
Know, and why, for the same substance, liquids have a slightly lower
density than the solid, but a much greater density than the gas.
Know and explain that liquids have no fixed shape, but have a surface and
a fixed volume at a given temperature and adopt the shape of the container.
Know and explain why liquids are very difficult to compress to a smaller
volume compared to gases because of the relatively small spaces between
Know and explain that weak forces between liquid particles allows a
liquid to be a fluid and flow freely e.g. through a pipe.
Know that the thermal energy, the internal energy of a gas is the kinetic
energy of the liquid particles.
Be able to explain the effect of changing temperature in terms of the
kinetic particle model of a liquid e.g. increasing temperature increase in
the average kinetic energy of the liquid particles and causes a small
expansion of the liquid.
Know that unless a liquid contains ions, liquids 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 liquids e.g. the motion of suspended pollen grain particles in
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INDEX of all my notes on the states of matter
(GCSE level and advanced pre-university level
notes on the states of matter and their properties