Unit 3: particle model of matter

• Density


  Mass per unit volume
• Density depends on the spacing of the atoms in matter
• 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
  • The spacing between atoms increase x10, as the particles have lots of energy to move, so volume increases greatly and therefore the density decreases greatly compared to solids/liquids
• If questions involve change in state and ask for new volume/pressure, the mass is the same!
• Mass is conserved during a change of state
• If 20g of liquid evaporates, the gas produced will also weigh 20g
• Physical changes are reversible, and not chemical changes
• Physical changes retain the original properties when reversed
• Internal Energy


  Energy which is stored by particles (atoms and molecules) within a system
• Forms of internal energy

  • Kinetic Energy (vibration of atoms etc.)
  • Potential Energy (between the particles)
• Heating a system


  • Increases the energy the particles have
  • This either raises the temperature of the system
  • Or produces a change of state
• Specific Heat Capacity


  The amount of energy required to raise the temperature of 1kg of a substance by 1°C
• Specific Latent Heat


  The amount of energy needed to change the state of 1kg of a substance without a change in temperature
• Energy is absorbed when melting and evaporating and energy is released when freezing and condensing
• Sublimation is when solid goes straight to gas – "dry ice" (solid CO2 does this)
• Pressure


  The total force exerted by all of the molecules inside the container on a unit area of the walls
• Changing the temperature of a gas, held at constant volume


  Changes the pressure exerted by the gas (known as the Pressure law)
• Increasing the volume in which a gas is contained, at constant temperature

  Can lead to a decrease in pressure (known as Boyle's law), this is due to the reduced number of collisions per unit area
• Doing work on a gas

  Increases its temperature
• Adding more particles to a fixed volume

  • Increases the pressure
  • Transfers energy to the particles, heating the gas
• A fixed number of particles for a smaller volume

  • Increases the pressure
  • Increases the temperature as the kinetic energy of each particle increases