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STATES OF MATTER - properties of gases and liquids (fluids) and solids

5. A simple kinetic particle model of a liquid

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.

(c) doc b 5. The particle model of a 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.
    • If there were no intermolecular forces, liquids could not exist!
  • 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 together.
  • 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.

Using the particle model to explain the properties of a Liquid

  • Liquids have a much greater density than gases (‘heavier’) because the particles are much closer together because of the attractive forces.
  • Most liquids are just a little less dense than when they are solid
    • Water is a curious exception to this general rule, which is why ice floats on water.
  • Liquids usually flow freely despite the forces of attraction between the particles but liquids are not as ‘fluid’ as gases.
    • Note 'sticky' or viscous liquids have much stronger attractive forces between the molecules BUT not strong enough to form a solid.
  • 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.
    • Liquids seem to have a very weak 'skin' surface effect which is caused by the bulk molecules attracting the surface molecules disproportionately.
  • 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).
    • 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 gases.
    • 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 liquid metal.
    • For more details see 'electrolysis' and 'metal structure'.
  • Diffusion
  • Brownian 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.

Learning objectives for the 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 liquid particles.

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 water.

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