8P2 Particle Model

• Particles


  Atoms, molecules or ions
• Solids


  • Forces of attraction between the particles hold them together and keep them in place
  • The hotter a solid gets, the faster its particles vibrate. This means that solids expand when they are heated
• Liquids


  • There are some gaps, but liquids cannot usually be compressed or squashed
  • The particles of a liquid have enough energy to break free of some of the forces of attraction between the particles. So particles in liquids can move around and can move over each other, allowing liquids to flow and be poured
• Gases


  • The particles in gases are widely spaced and randomly arranged, meaning they can be easily compressed or squashed
  • The particles in a gas have enough energy to overcome the forces of attraction between the particles, so are free to move in any direction. They move quickly in straight lines, colliding with each other and the walls of their container
• The particle model explains the differences between solids, liquids and gases
• Changes of state


  1. Heating up - Absorbing energy (endothermic)

  2. Melting = solid to liquid

  3. Boiling = liquid to gas

  4. Evaporating = liquid to gas slowly, at temperatures below its boiling point

  5. Cooling down - Transferring energy to surroundings (exothermic)

  6. Condensing = gas to liquid

  7. Freezing = liquid to solid

  8. Sublimation = Some solids change directly into a gas without becoming liquid first
• Solid carbon dioxide is often called dry ice. It must be kept inside well insulated containers because when its temperature rises above -78 °C it sublimes, or sublimates, into carbon dioxide gas. This process is endothermic, so it can be used to keep other substances cold
• Increasing pressure in gas

  • When the particles collide with the walls of the container
  • If the temperature increases
  • If the volume of the container decreases
• Pressure


  Force exerted over an area
• A car tyre contains gas under pressure. This means that there are more air particles pushing on the inside of the tyre than on the outside of the tyre
• Decreasing the volume of a closed container containing gas causes the pressure inside to increase
• Particles in a gas

  • Are spaced widely apart
  • Can be easily compressed because of the spaces between them
  • Move at random, quickly and travel in straight lines
  • Collide with each other and with the sides of their container
  • Move faster when the gas is hot as the pressure is greater