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

7. A simple kinetic particle model of a solid to explain its physical properties

• WHAT IS THE SOLID STATE OF MATTER?
• WHAT ARE THE PROPERTIES OF A SOLID?
• HOW DO SOLID PARTICLES BEHAVE?
• How does the kinetic particle theory of solids explain the properties of solids?
• A solid has a fixed volume and shape at a particular temperature unless physically subjected to some force.
• The greatest forces of attraction are between the particles in a solid and they pack together as tightly as possible in a neat and ordered arrangement called a lattice.
• The particles are too strongly held together to allow movement from place to place but the particles vibrate about their position in the structure.
• With increase in temperature, the particles vibrate faster and more strongly as they gain kinetic energy, so the vibration increases causing expansion.
• More on the kinetic particle theory of an ideal gas

• Solids have the greatest density (‘heaviest’) because the particles are closest together.
• Solids cannot flow freely like gases or liquids because the particles are strongly held in fixed positions.
• Solids have a fixed surface and volume (at a particular temperature) because of the strong particle attraction.
• Solids are extremely difficult to compress because there is no real ‘empty’ space between the particles, so increase in pressure has virtually no effect on the volume of a solid.
• Solids will expand a little on heating but nothing like as much as liquids because of the greater particle attraction restricting the expansion and causing the contraction occurs on cooling.
  • The expansion is caused by the increased kinetic energy of particle vibration, forcing them further apart causing an increase in volume and corresponding decrease in density.
  • Although the expansion of a solid is due to the higher average kinetic energy of the particles and the more energetic vibrations, they are still held together by the intermolecular bonding forces (or much stronger strong ionic or covalent bonds), which restricts the expansion - this is not part of the kinetic particle theory!
• Diffusion is almost impossible in solids because the particles are too closely packed and strongly held together in a lattice. The immobile particles cannot move around because there is no random movement into ‘empty space’ for them to move through.
Its quite a different situation in gases and liquids where diffusion readily takes place because of the freedom of the particles to move around at random and 'bash' each other aside!
• Heat conduction in solids
  • Apart from metals, most solids are poor conductors of heat energy, energy which is due to the kinetic energy of the vibrating particles in the crystal structure – remember, unlike gases and liquids, the particles can't move around, they just vibrate about a fixed point.
  • Heat energy is transferred by 'hotter' higher kinetic energy vibrating particles colliding against 'cooler' lower kinetic energy vibrating particles so raising their kinetic energy and spreading the heat energy through the solid structure.
  • The density of solids and order of particles is are greater than liquids (particles closest together), so the density or rate of 'collision transfer' vibration is much higher, so solids are better heat conductors than liquids (and much greater than gases).
  • However, although most non-metal solids are poor heat conductors, metals are exceptionally good heat conductors because of the freely moving electrons that can carry the kinetic energy rapidly through the crystal structure.
  • For more details see 'metal structure'.
• Electrical conduction in solids
  • Electrical conduction requires the presence of free IONS or free ELECTRONS i.e. particles that can carry an electrical charge within a solid structure. Which of course is impossible in most solids (except metals) because ALL particles can't move around, so even solid ionic compounds cannot conduct electricity.
  • Most non-metal solids are poor conductors of electricity (good insulators), but there are important exceptions.
  • All metals are relatively good electrical conductors because of the freely moving electrons that can carry the electrical current rapidly through the liquid metal. For more details see 'metal structure'.
  • Graphite and graphene, forms (allotropes) of the non–metallic element carbon, are electrical conductors due to free moving electrons in the solid structure, a rare exception of conducting solids apart from metals.

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