Topic 3: Particle model of matter

• Solid

  • Particles are very close together by strong forces of attraction, arranged in a regular pattern
  • Particles vibrate about fixed positions but are not free to move around so shape is fixed
• Solid

  • Hard to compress as there is no room for particles to move closer
• Liquid


  • Particles are close together by weaker forces of attraction, not arranged in any pattern
  • Particles move around each other and spread out in all directions to fill the container
• Liquid

  • Hard to compress as there is little room for particles to move closer
• Gas

  • Particles are very far apart with almost no forces of attraction, not arranged in any pattern
  • Particles move randomly at a range of speeds in different directions and spread out in all directions to fill the container
• Gas

  • Easy to compress as there is space between particles
• Density

  Mass per unit volume of a substance
• Solids usually have a very high density because the particles are packed very close together, so solids have a lot of mass for their volume
• Liquids usually have a high density because the particles are packed close together, so liquids have a lot of mass for their volume
• Gases have a low density because the particles are very far apart so gases only have a small mass for their volume
• Regular objects

  Dimensions can be easily measured with, for example, a ruler
• Irregular objects

  Dimensions cannot be easily measured with, for example, a ruler
• Internal energy

  Total kinetic energy and potential energy stored by all the particles (atoms and molecules) that make up a system
• The kinetic energy stored inside a system is due to the movement of the particles, and the potential energy stored inside a system is due to the forces between particles and bonds between atoms in a molecule
• Heating a solid

  1. Internal energy increases

  2. Solid turns into a liquid (melting)
• Heating a liquid

  1. Internal energy increases

  2. Liquid turns into a gas (boiling)
• Cooling a gas

  1. Internal energy decreases

  2. Gas turns back into a liquid (condensing)
• Cooling a liquid

  1. Internal energy decreases

  2. Liquid turns back into a solid (freezing)
• Sublimation

  Solid turning directly into a gas
• When a change of state occurs, mass is always conserved, as no particles are being added or taken away
• Changes of state are physical changes not chemical changes, because is the change is reversed, the material recovers its original properties
• Boiling

  Liquid turning into a gas
• Evaporation

  Liquid turning into a gas only on the surface of a liquid, as only the surface particles have enough energy to turn into a gas
• Heating graph of a solid

  1. Temperature rises as energy of particles increases

  2. Reaches melting point when temperature stops rising as state changes from solid to liquid
• Heating graph of a liquid

  1. Temperature rises as energy of particles increases

  2. Reaches boiling point when temperature stops rising as state changes from liquid to gas
• Cooling graph of a gas

  1. Temperature drops as energy of particles decreases

  2. Reaches condensing point when temperature stops dropping as state changes from gas to liquid
• Cooling graph of a liquid

  1. Temperature drops as energy of particles decreases

  2. Reaches freezing point when temperature stops dropping as state changes from liquid to solid
• Specific latent heat

  Amount of energy required to change the state of 1kg of a substance with no change in temperature
• Specific latent heat of fusion

  Amount of energy required to change 1kg of a substance from solid to liquid with no change in temperature
• Specific latent heat of vaporisation

  Amount of energy required to change 1kg of a substance from liquid to vapour with no change in temperature
• Pressure of a gas

  Due to the particles colliding with the walls of the container that the gas is held in
• By colliding with the walls of the container, the gas particles exert a pressure
• Increasing pressure of a gas

  1. Increase the number of collisions per second or the energy of each collision of gas particles

  2. Can be done by increasing the temperature
• Temperature of a gas

  Related to the average kinetic energy of the particles
• At lower temperatures

  Particles have less kinetic energy, so there are fewer collisions per second and collisions with less energy, the gas has low pressure
• At higher temperatures

  Particles have more kinetic energy, so there are more collisions per second and collisions with more energy, the gas has high pressure
• The particle collisions cause a force which acts at right angles to the walls of the gas container, which causes the pressure of the gas
• If the volume of the container is increased

  Pressure is reduced, provided the temperature is constant
• Increasing the volume of the container increases the spacing between gas particles so they travel much further before colliding with the walls of the container, reducing the number of collisions per second between the particles and walls of the container, reducing pressure
• Pressure and volume of a gas

  Pressure is inversely proportional to volume