Unit 14 - Particle model

• Density

  Mass per unit volume
• Solids and liquids

  • Have similar densities as the space between particles does not change significantly
  • Liquids usually have a lower density than solids (main exception is ice and water)
• Gases


  • Have a far lower density as the spacing between atoms increases x10, as the particles have lots of energy to move, so volume increases greatly and therefore the density decreases greatly compared to solids/liquids
• Mass is conserved during a change of state
• Physical changes are reversible, and not chemical changes
• Heating a system


  1. The amount of energy the particles have increases

  2. Particles vibrate more

  3. The temperature of the system increases

  4. The system changes state
• Changes of state


  • Melting
  • Evaporating
  • Freezing
  • Condensing
  • Sublimation
• Specific heat capacity


  The energy required to raise the temperature of 1kg of a substance by 1°C
• Specific latent heat


  The energy to change the state of 1kg of a substance without a change in temperature
• Specific latent heat of fusion

  Energy to melt/freeze
• Specific latent heat of vaporisation

  Energy to boil/condense
• Insulation


  • Reduces the amount of energy lost as it is a poor thermal conductor
  • Use reflective coatings to reflect IR radiation (heat) back into the system
• Pressure of a gas

  • Particles in a gas move randomly in every direction
  • A fluid can be a liquid or a gas
• Increased temperature

  Increases pressure in a constant volume gas
• Absolute zero

  The coldest possible temperature at 0 Kelvin or -273°C, where particles have no energy and remain perfectly still
• Increasing the pressure of a gas

  Causes it to compress (have a smaller volume)
• Doing work on a gas

  Increases its temperature
• Adding more particles to a fixed volume

  Increases pressure and temperature as there are more collisions per unit time with the walls
• Decreasing the volume of a fixed number of gas particles

  Increases pressure and temperature as the particles gain more momentum from colliding with the moving walls