P3 - Particle Model of Matter

• Describe the 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


  Increases the pressure of a gas when held at constant volume
• Why pressure increases as temperature increases (at a 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 a low pressure, and gas B is at a high pressure


  • There are more collisions per second in gas B than in gas A
  • The rate of collisions is higher in B
• Force that the pressure of a gas exerts on the walls of its container

  • The net force acts at right-angles to the container's surface
  • The force increases as pressure increases
• Explain how increasing the volume of a gas results in a decrease of 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
• The product of pressure and volume for a fixed mass of gas at a constant temperature is constant
• Unit used 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 the temperature of air inside a bike pump increases when it is 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
• 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
• 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)
• 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
• Explain why decreasing the temperature of a fixed volume of gas decreases its pressure (3)

  Decreasing the temperature of the gas means that the gas particles have less energy in their kinetic energy stores. They collide with the container less often and exert a smaller force when colliding so gas pressure is low
• 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
• What is always conserved when a substance undergoes a change of state