Particle model of matter

• Density


  The mass per unit volume of a material
• Density equation


  1. ⍴=m/v

  2. Density (kg/m³)

  3. Mass (kg)

  4. Volume (m³)
• States of matter in order of increasing density

  • Gas
  • Liquid
  • Solid
• Solid


  • Tightly packed in a regular arrangement
  • Particles can only vibrate on the spot
• Liquid


  • Close together, but with an irregular arrangement
  • They can flow over each other
• Gas


  • Separated, with no regular arrangement
  • Particles can move freely
• Mass is always conserved when a substance undergoes a change of state
• Change of state


  The material can return to having its previous properties if the change is reversed
• Chemical change


  The material cannot return to having its previous properties
• Sublimation


  When a solid changes into a gas without passing through a liquid state
• Evaporation


  When a liquid changes into a gas state
• Condensation


  When a gas changes into a liquid state
• Water boils in an open pan

  Some of the water will evaporate and turn into water vapour, leaving the pan and decreasing its mass. The mass of the whole system however remains constant.
• Bathroom mirror mists up

  1. Hot water evaporates to form water vapour

  2. This water vapour lands on the cooler mirror

  3. The vapour condenses and returns to liquid state on the mirror's surface
• Internal energy of a substance

  The energy stored by the particles

  The sum of the total kinetic and potential energies that make up the system
• Heating a substance


  Transfers energy to the substance

  Increases the energy of the particles that make up the substance
• What heating a substance can do

  • Raise its temperature

  Change the state of the substance
• Factors that determine temperature change of a system

  • Mass of substance being heated

  Type of material (Specific heat capacity)

  Energy inputted into the system
• Equation to calculate temperature change when a substance is heated

  ΔE = m c Δ𝛉

  Energy (J), Mass (kg), Specific Heat Capacity (J/kg/°C), Temperature (°C)
• Specific heat capacity

  The amount of energy needed to increase the temperature of 1kg of a substance by 1°C
• Changes of state


  The internal energy of the substance will be increased or decreased

  The temperature of the substance will remain constant
• Specific latent heat


  The amount of energy needed to change the state of 1kg of a substance with no change in temperature
• Equation for energy required to change state

  Energy to change state = mass x specific latent heat

  Energy (J), Mass (kg), Specific latent heat (J/kg)
• Specific latent heat of fusion

  The energy required to change 1kg of a substance from solid state to liquid state without a change in temperature
• Specific latent heat of vaporisation

  The energy required to change 1kg of a substance from liquid state to gas state (vapour) without a change in temperature
• Motion of molecules in a gas

  They are in constant random motion
• Factor affecting average kinetic energy of gas molecules

  • The temperature of the substance
  • The higher the temperature, the higher the average kinetic energy of the molecules
• Increasing temperature of a gas held at constant volume

  Pressure of the gas will increase
• Why pressure increases as temperature increases (at constant volume)

  1. Kinetic energy of molecules increases

  2. Collisions between molecules becomes more frequent

  3. Greater rate of change of momentum

  4. Greater force and therefore pressure
• If gas A is at low pressure, and gas B is at high pressure


  • There are more collisions per second in gas B than in gas A
  • The rate of collisions is higher in B
• Force exerted by pressure of gas on container walls

  • The net force acts at right-angles to the container's surface
  • The force increases as pressure increases
• How increasing volume of gas results in decreased pressure

  1. Molecules become more spread out and so time between collisions increases

  2. This reduces the rate of collisions

  3. Rate of change of momentum decreases, and so force exerted on container decreases, resulting in a lower pressure
• For a fixed mass of gas at constant temperature, the product of pressure and volume is constant
• Unit for pressure


  Pascal (Pa)
• What increases when you do work on a gas

  • The internal energy of the gas
  • This can also lead to an increase of temperature
• Why temperature of air inside a bike pump increases when pumped

  1. Work is done on a gas when it is compressed

  2. Doing work on a gas increases its internal energy, so also increases the average kinetic energy of the molecules

  3. Temperature increases with an increase of average kinetic energy