P6 Molecules and 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
• Change of state


  • Mass is conserved
  • Physical changes are reversible, and not chemical changes
  • They are not chemical because the material retains its original properties when reversed
• Changes of state


  1. Melting

  2. Evaporating

  3. Freezing

  4. Condensing

  5. Sublimation
• Internal energy


  • Energy which is stored by particles (atoms and molecules) within a system
  • Takes the forms of kinetic energy (vibration of atoms etc.) and 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
  • Specific Latent Heat of fusion is energy to melt/freeze
  • Specific Latent Heat of vaporisation is energy to boil/condense
• 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)
• Temperature changes of ice


  1. Solid

  2. Reaches 0 degrees celsius

  3. Melting

  4. Liquid

  5. Boiling

  6. Gas
• 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)
• Changing the volume of a gas

  • Affects the pressure
  • Increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure (known as Boyle's law)
• Doing work on a gas

  Increases its temperature
• Adding more particles to a fixed volume

  Increases the pressure and temperature of the gas
• A fixed number of particles for a smaller volume

  Increases the pressure and temperature of the gas
• Density


  Data: Density (ρ) = Mass (m) / Volume (V); Example: 500 g / 0.5 L = 1000 g/L or 1 g/cm³.