Topic 14 - Particle Model

• Density

  Mass per unit volume
• Density formula

  density = mass / volume
• Density units

  kgm^-3
• Density depends on the spacing of the atoms in matter
• Solids and liquids have similar densities because the space between particles does not change significantly
• Usually liquids have a lower density than solids (main exception is ice and water)
• Gases have a far lower density compared to solids/liquids due to the spacing between atoms increasing x10
• 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 during changes of state are reversible, and not chemical changes
• Heating a system increases the amount of energy the particles have, makes particles vibrate more, and increases the temperature of the system or changes its 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 Heat Capacity formula

  energy = mass × specific heat capacity × temperature change
• Specific Heat Capacity units

  J/kg/°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 is the energy to melt/freeze
• Specific Latent Heat of vaporisation is the energy to boil/condense
• Energy is absorbed when melting and evaporating, and released when freezing and condensing
• Insulation

  Thermal energy transfers out of any system, using thermal insulators reduces the amount of energy lost
• Particles in a gas move randomly in every direction
• A Fluid can be a liquid or a gas
• Pressure formula

  pressure = force / area
• Pressure produces a net force at right angles to any surface
• Particles exert a force on the wall when colliding with it, changing momentum during their collision
• Temperature and Pressure (in a constant volume) increased
• Particles exert force at right angles to any surface

  1. Particles collide with a wall, changing velocity

  2. Particles change momentum during their collision

  3. Particles exert a force on the wall (as force = ∆momentum / time)
• Increased temperature

  More energy given to the particles
• Temperature and Pressure (in a constant volume)

  1. Increased temperature leads to more energy given to the particles

  2. Thermal energy is transferred to kinetic energy

  3. Particles move at faster speed

  4. Collisions with walls occur more often

  5. Particles hit the wall with greater impact

  6. Pressure increases
• Absolute Zero is 0 Kelvin, or -273°C
• Nothing can exist at a colder temperature than Absolute Zero, the coldest possible temperature
• Particles at Absolute Zero have no energy, so they do not vibrate at all, they remain perfectly still
• Converting Kelvin to Centigrade

  1. T kelvin = (T − 273) centigrade

  2. 4K = −269°C

  3. 0°C = 273K
• Pressure changes (Physics Only)

  • Gases want to remain at a constant temperature
  • Increasing pressure causes gas to compress
  • Pressure ∝ 1/volume (inversely proportional)
• For a gas at fixed mass and temperature: P1V1 = P2V2
• Doing Work on a Gas (Physics Only)

  1. Doing work on a gas increases its temperature

  2. Work done = Pressure × Volume
• Adding More Particles to A Fixed Volume

  1. Pumping more gas into the same volume increases pressure

  2. Energy is transferred to the particles when more gas is added into the fixed volume, heating the gas
• A Fixed Number of Particles with A Decreasing Volume (Physics only)

  1. Particles gain momentum as they collide with the wall moving inward

  2. Pressure increases as particles collide with walls more frequently

  3. Temperature increases as kinetic energy of each particle increases