CIE A Level Physics (9702) 2019-2021

Revision Notes

18.1.1 Kinetic Model for Matter

Kinetic Model for Matter

  • The kinetic model of matter states that:

Matter is composed of a large number of small particles that are in constant motion

  • This can be used to explain:
    • The structure of solids, liquids and gases
    • Why melting and boiling take place without a change in temperature
    • Why the specific latent heat of vaporisation is higher than specific latent heat of fusion for the same substance
    • Why a cooling effect accompanies evaporation

The Structure of Solids, Liquids & Gases

  • In a solid:
    • The molecules are very close together and held in place by strong intermolecular bonds
    • These bonds prevent the molecules from moving, giving solid their rigid shape
    • The atoms are arranged in a regular pattern
    • The molecules vibrate about fixed positions
  • In a liquid:
    • The molecules are still close together (no gaps) but are no longer arranged in a regular pattern
    • The molecules have enough energy that they are able to break the intermolecular bonds between them
    • As a result, the molecules can move around and able to slide past each other, allowing the liquid to flow
  • In a gas:
    • The molecules are widely separated – about 10 times further apart in each direction than in liquids
    • The molecules move about randomly at high speeds
    • The molecules have no intermolecular forces between them
    • As a result, the molecules are able to move freely
    • The large spaces between molecules mean gases can easily be compressed and expanded

Why Melting & Boiling Take Place Without a Change in Temperature

  • Melting is when a solid turns into a liquid eg. ice melting into water
  • Boiling is when a liquid turns into a gas eg. water boiling to water vapour
    • In both cases, the water must absorb energy in order to melt or boil
  • This energy is not used to change the temperature of the water, only to change its state
    • This is because the energy is needed to break the bonds between molecules

Why Latent Heat of Vaporisation is Higher than Latent Heat of Fusion

  • Vaporisation is when a substance changes state from liquid to gas
  • Fusion is when a substance changes state from solid to liquid
  • In the vaporisation process:
    • Energy is required to completely separate molecules to the point where intermolecular forces are no longer acting
    • This requires much more energy than the separating molecules from a solid to a liquid
  • In the fusion process:
    • Energy is required to separate molecules from a rigid arrangement into a freely-flowing arrangement
    • This requires less energy as the intermolecular bonds become weaker, but do not completely break

Why Cooling Accompanies Evaporation

  • Evaporation requires heat energy to convert a liquid into gas since the intermolecular bonds in a liquid must be broken completely
  • This energy is removed from the molecules as they change state, and hence taken away from the surface of the liquid as the gas moves away, hence causing a cooling effect
    • This is why, for example, a person may feel instantly cooler when getting out of a hot shower or swimming pool

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